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It is hard to survive as dragon - and yet they do. Despite being known for their nasty claws+bite, fear radiance and breath weapon - thus becoming a high priority target for nearly any ranged attack - they survive past eight centuries. How is this done?

Any long-lasting dragon would fathom bounded accuracy and 150'/600' range of bows. For example, a single-accidental pass of a small yet well-hidden community of wood elves ('longbow proficiency + high dexterity = +4 to hit') requires an AC of 25+, lest this wyrm suddenly become a pincushion.

How does a dragon increase their armour class? Possibilities abound: barding (need it be mithril for flight)? A shield (would it stack with their regular AC)?, retro-fitting magic plate armour (can a dragon wear dragon-scale armour? would-could-should they have a chest plate fitted)? What magic items fit 'gargantuan'? The list goes on / i have no idea where to begin... i just want a better armour class for a Great Wyrm.

I was hoping this would gain some kind of general answer rather than a slew of over-simplistic questions ('Could a dragon wear boiled leather barding - or must it be supple due to flexibility required for dragon-flight???' etc.)

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    \$\begingroup\$ You cite being "a high priority target for nearly any ranged attack". But then you say the only countermeasure that's acceptable to deal with them is raising your AC. Which is it? \$\endgroup\$ – Stop Being Evil Sep 19 at 3:16
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    \$\begingroup\$ Using the mob rules for simplicity (roll of 15 needs 4 attacks to hit once), with an average damage of 7.5 per shot, you need about 137 archer-turns to take down an Adult Red Dragon, assuming close range. But between Frightful Presence and long range most attacks will be with disadvantage, which averages an effective -4 penalty, which makes 10 attacks to hit once, so it goes to 341 archer-turns. Ignoring cover. With +13 Perception, hiding 300+ battle-ready archers is non-trivial. Although the point of a dragon wishing to increase their AC still stands. \$\endgroup\$ – BBeast Sep 19 at 3:33
  • \$\begingroup\$ @StopBeingEvil I had assumed that raising AC was the only solution. Please feel free to correct me. I briefly thought about stuff like Mirror Image, but that is only good for a few hits. What else did you have in mind? \$\endgroup\$ – Tim of Time Sep 19 at 3:40
  • \$\begingroup\$ @BBeast In our world's history, armies could easily be many thousands of people - i have no clue how many would be in-range archers. Needless to say, doing a breath-blast fly-by would be a disaster without extremely good protection. True though, actually hiding an army is quite hard to do in D&D terms or in reality. \$\endgroup\$ – Tim of Time Sep 19 at 3:44
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    \$\begingroup\$ @TimofTime Indeed, even an ancient dragon would have a bad day if they tried to solo an entire army without a substantial advantage (such as increased AC or some decent cover). But "a single accidental pass of a small yet well-hidden community of wood elves" does not equal facing an army of archers standing with bows at the ready all with clear shots at the dragon. But I'm just nit-picking the specific example. Your general point that a modestly large group of archers is a serious threat to a dragon still stands. \$\endgroup\$ – BBeast Sep 19 at 7:45
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Eternal vigilance is the price of liberty

Title of question asks for AC improvement, but OP says in comments "I had assumed that raising AC was the only solution. Please feel free to correct me." My answer will not touch on AC.

OP posits a "a single-accidental pass of a small yet well-hidden community of wood elves"

I take the 'accidental' and 'well-hidden' to mean that the dragon is flying over the elves without prior knowledge that they are there.

PHB: "Combat"

The GM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the GM compares the Dexterity (Stealth) checks of anyone Hiding with the passive Wisdom (Perception) score of each creature on the opposing side.

Any time the dragon needs to get somewhere, it should be flying at altitude - above darkvision range and preferably above bow range. If for some reason it is forced to fly near the ground, it should choose to move stealthily. Even the youngest dragons have bonuses to Stealth and these improve as it ages.

The wood-elf community, on the other hand, is stationary and its members are unlikely to be continually taking the Hide action 24/7 as they go about their lives. Thus the dragon should likely notice the elves and react accordingly.

If the dragon is flying in the day, it will likely see them while still outside of bow range and can simply avoid the community. If terrain or darkness result in the dragon being within bow range by the time it notices the community, it should immediately move outside of bow range (Dash if necessary), or, if that is impossible, immediately take the Dodge action while it retreats. Since it is moving stealthily and at least some of them are not, it probably will be able to see them first, achieve surprise for many of them, and extract itself from the situation before they can bring numbers to bear.

A more challenging example would be a group of elven ambushers intentionally Hiding near where, for some reason, the dragon must pass at some point within bow range. Then, it is possible that the elves might be aware of the dragon's presence before it is of theirs and they might achieve surprise. Fortunately for the dragon, it has a good to great Perception score and might have the Legendary Action Detect. Dragons should be difficult to surprise. The more ambushers there are, increasing the threat to the dragon, the more likely that at least one of them will get a Hide score below the dragon's Passive Perception, and it will be warned.

Note on other answers: I do appreciate the time taken to do the combat calculations for ancient red dragons. But the OP asked how do they survive past eight centuries - so presumably they are interested in the younger age classes as well.

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  • \$\begingroup\$ True on young-dragon concerns. Ten 'apprentices' (CR 1/4) with Magic Missile do 220 damage - in two rounds, on any AC. Poor 'Young' dragons: no fear-aura / large / obvious breath weapon trail / easily tracked. Those FLYING are targets to everything (especially other dragons). Entire kingdoms fear 'adult' dragons, those attaining a century of age - Hunting drakes would be praised... and so highly rewarded. Only dragons with a clan of Tucker's Kobolds stand a chance. Plate mail would not help at all! \$\endgroup\$ – Tim of Time Sep 20 at 0:00
  • \$\begingroup\$ Note that the wood elf community/ambush rolls stealth as a group per the DMG rules-- they become more likely to get the average result over time, not more likely to fail (and given their probable forest-hiding-in bonuses that's more likely to succeed against most dragons) \$\endgroup\$ – Please stop being evil Sep 26 at 18:01
  • \$\begingroup\$ @Pleasestopbeingevil I don't know where the DMG discusses group checks - but if you mean the group check rules on PHB 175, then yes treating the ambushers as a group would tend to make their ability the average (not over time, since an ambush roll would be for an instant, but certainly over the group) and diminish the effects of singular low rolls. But, first, "Group checks don’t come up very often, and they’re most useful when all the characters succeed or fail as a group." I think it can be argued that an ambush doesn't succeed or fail as a group - if the dragon spots any of the elves \$\endgroup\$ – Kirt Sep 26 at 18:35
  • \$\begingroup\$ then the ambush fails - their success is limited by the single worst ambusher. Second, the passage explicitly says "When a number of individuals are trying to accomplish something as a group, the DM might ask for a group ability check." Since the OP is a DM attempting to preserve dragons without changing fundamental rules, simply deciding that a group check is not called for in this instance is their perogative. \$\endgroup\$ – Kirt Sep 26 at 18:38
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Frame challenge: Just raise its AC.

If you’re the DM and want your dragon to be more of a challenge or have better survivability, modify its stats. Raise its AC or give it more HP.

There is no guidance found in any sourcebook about giving a dragon armor, but the Dungeon Master’s Guide encourages the DM to modify statblocks to suit their needs. So just do that. And if you want to make your dragon particularly cool or menacing, just say it has armor. You don’t need rules to do this, you’re not a player, you’re the DM. It’s your world, and if you want dragon armor in your world, then have dragon armor in your world.

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The barding rules state that any armor can be barding, but they're unfortunately quite vague on which creatures are proficient with barding -- see Are mounts proficient in armour (barding)? for more on this topic. If your DM rules that dragons are proficient with armor, then a dragon can wear a +3 plate armor and have a 21 AC. The Adult Red Dragon has a 19 AC normally, so this helps a bit.

There are no rules that suggest that a monster can wear a shield ("shield-barding"?). We might imagine that a creature would need real arms to wear a shield, and not (eg) forelegs or wings. If your DM nonetheless rules that a dragon can wear a shield, you can factor that into the dragon's AC.

It's not clear whether dragons can wear armor while flying. The aaracokra can only wear light armor while flying; the winged tiefling can wear light or medium armor while flying. Your DM will have to issue a ruling on how much armor a dragon can wear while flying.

A ring of protection will give a dragon +1 AC.

Most dragons are not spellcasters, but a dragon might have an ally who is a spellcaster. The spell shield of faith grants +2 AC. The spell haste grants +2 AC as well as other benefits. The spell greater invisibility gives attackers disadvantage. If a dragon is a spellcaster, it can cast one of these spells on itself; dragons have pretty good Constitution saves, so it's unlikely to fail at concentration.

Most dragons have Frightful Presence, which can also give attackers disadvantage.

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  • \$\begingroup\$ An ancient Red has a frightful presence of 120' and the short range of a bow is 150'. What's more: on a successful save the fear is GONE for the day. So unfair! The rest is good, though i wonder if a dragon could use a shield whilst flying (kite perhaps?). \$\endgroup\$ – Tim of Time Sep 19 at 3:49
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As has been pointed out, you're the GM so you get to say how this works. I'd probably just tweak the stats a big directly instead of trying to use items (because that approach lets you tweak the treasure independently of the stats, which is in and of itself important, just the raw materials in a full set of plate armor for a grown dragon is likely to be worth at least 10k gold even ignoring any value of the armor itself).

But AC is probably not the stat you should tweak here

AC effectively acts as a multiplier on the base HP values proportionate to how likely the attacker is to hit the target, because you need to make more attacks to deal the same amount of damage. The exact formula for this is 1 / chance to hit. Ignoring the possibility of advantage and disadvantage (which skew these numbers significantly), you get:

Minimum Roll to Hit | Effective Multiplier
--------------------+---------------------
20                  | 20
19                  | 10
18                  |  6.666...
17                  |  5
16                  |  4
15                  |  3.333...
14                  |  2.875...
13                  |  2.5
12                  |  2.222...
11                  |  2
10                  |  1.818...
 9                  |  1.666...
 8                  |  1.538...
 7                  |  1.428...
 6                  |  1.333...
 5                  |  1.25
 4                  |  1.176...
 3                  |  1.111...
 2                  |  1.052...
 1 (always hits)    |  1

It should be immediately obvious from this that increasing an already high AC has a much bigger effect on how many hits it takes to kill the target than increasing a low AC (and, conversely, increasing the chance to hit a target has diminishing returns). This asymmetry exists in any system that uses AC rules equivalent to 5e (including anything derived from the 3.5e or 5e SRD), as well as systems that use % damage reduction instead of AC (which includes a very large number of video games).

The net effect of this is that you have much more precise control of how 'tough' your creature is by tweaking HP instead of AC (especially for creatures with both stats high or both stats low).

Of course, there's also another factor to consider, most players can only get up to a +14 modifier on an attack without magical assistance (+6 prof with weapon at level 20, +5 on the modifier, and a +3 magic weapon, barbarians can instead get +16 on melee at level 20 due to the STR boost from their capstone). This means, based on the above table, that against fully kitted top level players, an ancient red dragon still has more than a 1.5x multiplier on it's health due to it's 22 AC, and therefore an effective HP of more than 816 if you're using average HP. That's an insane amount of HP to have, even with optimal weapons and average damage that's still at least a 20 round combat to kill it with a party of four.


However...

You're making a lot of assumptions here about how 'at risk' the dragon would be. Looking at your original scenario, only about 15% of the shots made by the elves would actually hit an ancient red dragon, and if 100 of them make a single shot each, that's an average of about 67-68 damage they would deal. That's a non-negligible chunk of damage, but the ancient red dragon has a d20 hit die, and it has 28 of them, which means on average this is a drop in the bucket for it (in most cases it will take it at most one day to recover).

Even if we assume max damage on each shot, a group of elves with regular longbows would actually need 460 elves taking a single shot each to kill an average ancient red dragon. In reality, you actually need more than five thousand to ensure a kill if you are actually rolling for HP for the dragon instead of using the average (max HP is 812, so you need 5414 such elves to ensure a kill if you can assume that at least 15% will hit).

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