What's my most efficient use of spell slots to help my AC?

Question

I've got a wizard with 11 Dex and I'm trying to figure out what my most efficient use of spell slots is to help me from getting hit.

We typically have 2-4 very difficult encounters per day and I want to balance my spell slot uses where I can provide as much firepower as needed. The monsters are generally higher CR as opposed to mobs. It's usually approximately 1:1 in terms of PCs:NPCs and if it isn't, it's fairly close. Mob fights have been much less common.

Spending slots on defense with potentially diminishing returns due to the actual AC values is where I am looking to be most efficient.

Currently 5th level and have the following spells available:

1st level - mage armor and shield
2nd level - mirror image

Scenario A is casting mage armor in the morning and having AC 13 throughout the day. The difference between 10 and 13 seems minimal and that i'm likely going to get hit no matter what. But casting shield will boost to 18 which is legitimate. But then I've burned 2 spell slots on this method.

Scenario B is casting mirror image in a fight I think I'll need it. Leave my AC at 10 and hope the images take the hits. Can still use shield if AC 15 will save me at some point.

I'm open to other scenarios as well, just trying to figure out the optimal strategy to get the best return on my spell slot usage for defense vs offense.

My offensive strategy

Most of my offensive spells are evocation and that's where I'm mostly going to spending my spell slots. I'll have a couple of control type spells that aren't, but a lot of the instant-damage I do will be evocation.

While I try to stay away from enemies and not be in melee range, it's going to happen at times. It's those times (and ranged attacks), that I'm most concerned about keeping myself alive after.

Answer 1

These spells have durations.

Mage armor has a duration of 8 hours and does not require concentration.

Mirror image has a duration of 1 minute and does not require concentration.

Shield has a duration of 1 round and does not require concentration.

You could convince another party member to cast shield of faith on you.

Because none of these requires your concentration, you can use them in parallel. Do not discount unlimited +3 AC from mage armor for 8 hours. As far as efficiency goes this is solid, and it will happen more often than you're thinking.

You are not a tank.

Stacking AC pays dividends, but the best way to avoid damage is to be tactical. Stay out of the fray. Remove enemies from the fray. Spells like fog cloud and grease can make it hard to get to you. Not as universal, but protection from evil and good grants disadvantage as well as some status protection.

If you have people who can leverage ranged attacks, fear could really turn the tide for you, since frightened creatures cannot move closer to you.

Being a wizard means being prepared for what you are going to face. Mage armor is worthy of a spell slot first thing in the morning. What you prepare and cast afterwards depends on whether your foes are melee-dependent, ranged attackers, or able to force saving throws. Whatever they have coming for your group, get out of its way.

Answer 2

Don't rely on AC for defense

AC is great, if you are sitting there being hit all the time. But that isn't a very good strategy for a wizard. Wizards aren't tanks, they are tricksy magic users. There is a lot you can do with cantrips, movement, and actions.

Here are 2 tactics to avoid damage, that do not rely on AC:

• Positioning
• Control

Positioning

Make sure you are standing in an easily defend-able location. Generally this means keeping danger far away, standing somewhere that is hard to approach, and standing somewhere that makes you hard to target.

• In a house, looking out the window.
• In a tower above the battle.
• Standing at the back with your party's fighter between the enemy and you.
• Standing behind a big rock.
• Anchoring your flanks with impassable terrain such as a wall, or difficult terrain such as a river.
• Stand behind anything that blocks line of sight to anyone you aren't attacking: a high backed chair, your barbarian ally, lay behind a rock.

Control

No matter how good your positioning, you will probably have some vulnerability. Perhaps the tank is ahead of you, and a wall to your left, but to your right an enemy could sneak through. So long as enemies have someone better or easier to fight, they will probably go after them instead. How can we prevent the enemy from finding us appealing or from exploiting this weakness?

• Push a table or chair in the way to create difficult terrain
• Use a cantrip like Bonfire to create a hazard
• Use Booming Blade to make it unappealing for enemies to chase you down
• Use Shocking Grasp to escape
• Throw some caltrops or ball bearings on the ground
• Use Shape Water, Mold Earth, and Control Flame to create environmental hazards
• Use Prestidigitation and Minor Illusion to create distractions or imagined hazards
• Use Gust as a readied action if you are at risk of being approached
• Use Frost Bite to weaken enemies who harass you
• Use Ray of Frost to prevent enemies from approaching

Some good ol' normal actions

People often overlook the actions you can take in combat. They are very powerful defensive options. These fit under denial and positioning, but they deserve special mention.

• Dash: If you run far away, the enemy needs to catch up to you. If you both have speed of 30ft, then they will have to use their action just to get close to you. Before long they will give up and find an easier target. For a bonus, run past your friend so the enemy will have to take a detour, or risk an opportunity attack.
• Disengage: A get out of jail free card. Hopefully your party is nearby so you can drag the enemy past your allies for opportunity attacks!
• Dodge: Disadvantage on attacks you can see is roughly equal to -5 to hit, when the enemy to hit is 2 or less, -4 if the enemy to hit is 4 or less, -3 if enemy to hit is 6 or less, etc.* If necessary, you can still shield as a reaction: dodge and shield stack very nicely, especially on top of Mage Armor (13+5 = AC 18), giving you a good chance of surviving a bad situation. Like shield, dodge works on all attacks until your next turn, for no spell slots! Also gives you advantage on Dex saves.
• Hide: Not just for rogues, anyone can duck behind a crate and instantly force enemies to guess their location (potential automatic dodge), and force enemies to have disadvantage even if they guess correctly. If you can cast spells without vocal components or attack rolls you can even do that without giving away your location!
• Ready: This is great in combination with any number of spells. Someone approaches within 10ft? Move away 30ft, or Ray of Frost them. Someone comes in contact? Gust them back. Someone attacks you? Chill touch them.
• Use an Object: There are a lot of cheap utility items that anyone can use. Ball Bearings can knock someone prone, or make them walk slowly. Flasks of Oil work well with fire spells, a waterskin with Shape Water makes a patch of ice, rope or caltrops can create deterrence. And that's only mundane items.
• Go prone: Attacks from more than 5ft away have disadvantage. Lay on the floor and enjoy the equivalent of -5 to hit to all ranged attackers. Don't do this if an enemy is nearby that can walk over and melee you (geting advantage).

As a crafty wizard you have a boat load of options. Leave tanking to the tanks.

*Note: At 10 AC, the effectiveness of dodge depends on the enemy's hit chance. If the enemy's hit bonus is less than 3, your defensive abilities are equivalent to about AC15, if the hit bonus is <5, you are equivalent to AC 14, with <7 it's AC 13, with <8 AC 12, with <9 AC 11, and above that, you are back to AC 10. Although keep in mind that no matter the enemy hit chance, the odds of a critical hit are reduced from 1 in 20 to 1 in 400.

Answer 3

Most Efficient Spell for AC:
Mage Armor, if you get attacked a lot over the course of the day.
Mirror Image, if you get attacked a lot over the course of a combat.
Shield, if you get attacked a lot over the course of a round.

A "hard" encounter for a group of five level 5 characters, with a 1:1 ratio of monster to PC, is about CR2 for each monster.

I'll overestimate the difficulty and call it CR4 for each monster for the purposes of this answer.

That means your average opponent has +5 to hit and deals ~30 damage per round, usually split into two attacks for ~15 damage each.

Against AC10 (you spend no slots on defense): 80% chance to be hit, or 12 expected damage per attack. This is your baseline.
AC13: 65% chance to be hit, 9.75 expected damage - saving 2.25 damage per attack.
Shield: 25% chance to be worth using, saves 15 damage immediately. Reduces followup attacks by 3.75 damage per attack.
Mirror Image: prevents 45 damage at the risk of 16.5 damage to yourself (calculated using the chance an image doesn't take the hit each time it's possible)

Takeaways:
Mage armor's effectiveness depends on how many times you get attacked during its duration. If you get attacked 6 times over the course of the day, its efficiency passes shield (when Shield is used against only a single attack).
Mirror Image carries about double the efficiency of Shield in any fight where it is fully utilized. Unless you're getting attacked 13 times per day, it beats Mage Armor in efficiency.

Best plan though: Getting a monster to spend one round attacking anyone other than you. That's worth 1.5 casts of Shield, and often costs no spell slots.

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One interesting note is that the number of attacks it takes for Mage Armor to equal Shield holds up regardless of your starting AC and the monster's starting attack bonus (as long as they need more than 1 and less than 20 on the die to hit).

So really, any day you expect to get attacked 6+ times, you should cast Mage Armor if you're willing to cast Shield during combat.

Answer 4

Look at party efficiency, and consider Sanctuary from your Cleric

I will recommend that you ask the Cleric to cast Sanctuary on you before some fights, and that you use your spells to mostly buff your allies when Sanctuary is up. (mostly buff, see the caveat at the end).
Reducing the number of possible attacks on you at all is a way to improve your defense. (I am aware that you are a veteran of 5e battles, so I apologize if that is too basic of a point). The time will come in any fight for dropping Sanctuary and letting loose with damage, but I have found with wizards that this is a "pick your spots" decision in days with multiple battles.

If you don't get attacked, you don't need armor class

Unless the enemy has a pretty substantial Wisdom save bonus, most attempts to attack will result in an attack on someone else, per Sanctuary's main feature. Who needs armor class if they are not being attacked?

Granted: you are in tier 2. This may not last all the way to Tier 3 depending on which monsters you run into and which ones have good Wisdom save benefits.

And the usual supplemental plan: always save one or two slots for shield.

A single casting of Mage Armor, either before or after the short rest that gives you Arcane Recovery, you'll want to balance with when you have Sanctuary up, as it does not have infinite duration. Mage armor is higher risk since in Tier 2 AC 13 isn't that hard for monsters to get through. (All said and done, I don't think there's a one size fits all solution to your problem).

Once per day when you finish a short rest, you can choose expended spell slots to recover. The spell slots can have a combined level that is equal to or less than half your wizard level (rounded up), and none of the slots can be 6th level or higher. {2.5 rounded up = 3)

You get three spell slots back each day during that one recovery period. I suggest that you save one for you, and two for the party's needs. (That's what my brother did).

Why do I suggest this?

We tried it in one of our first 5e groups, with my Tempest cleric casting Sanctuary on our Wizard. It was uncanny how often an attack on our Wizard was foiled by that spell. Granted, our Wizard was a mix of damage and control, and I tended to be the Bless Bot, so that our martials would keep hitting the enemy, I blessed them; and the Wizard tended to slow down or debuff the enemy. (And maybe we were just supremely luck with the rolls, not sure).

Caveat: this was a pre-2018 errata approach

This approach would tend to focus your spells cast as buffs (like haste) for your other characters rather than direct damage spells. We did this some years before the errata of 2018, and our DM ruled that the control spells like Web and Slow and Fog Cloud weren't attacks.

The implementation can get tricky, such as if you cast web or slow, and then someone casts sanctuary on you (initiative order permitting); you can concentrate for a few more rounds without casting a spell that effects the enemy, but your DM might look at this combination as approximating cheese.

Discuss with your DM on their ruling.