Is there a good formula/strategy for making monsters harder as characters level up without just throwing more of them? My kids, for example, love Kobolds but I just don't want combats of 30 of them.
You can also make them smarter and more resourceful. Two levels ago, say five kobolds were a challenge. Now, consider:
- Five kobolds who have hired an ogre to protect them against the recent plague of adventurers
- Five kobolds who have raised a bear from a cub and trained it to help them
- Five kobolds who have scavenged a wand or two
- Five kobolds who have laid a cunning trap
- Five kobolds who are carrying around magic mushrooms that give a random beneficial effect when consumed (breathe fire, fly, stoneskin, enlarge, what-have-you)
- Five kobolds who have taken hostages
- Five kobolds who have poisoned their weapons
- Five kobolds who have commissioned an incriminating oil painting and will show it to the gendarmes if provoked
- Five kobolds who happen to be especially well prepared for the types of damage the characters cause
None of that is to say that just boosting there stats isn't also a great option, and one that's especially easy in 4.0. But some variety can help stave off the feeling that no matter how strong the characters get, the kobolds just match them stat for stat.
I have a chart I used to individualize common monsters and make them more dangerous/effective. Some suggestions I would give:
A leader type with more HD and more intelligent (better tactics, etc)
Better or magical weapons, especially "throw away" weapons like arrows or bolts
Missile weapons...even a thrown hand axe, club or spear can really even the odds if there are 20 of them flying at a character!
Better/more armor....specially made armor (that the characters can't utilize, of course!) that give them a much better AC bonus.
Use the terrain! Perhaps the caves with the kobold lair are only 5 feet tall, giving PCs a huge disadvantage. Or maybe it's in a no-magic zone, and spells don't function. Perhaps it's in a cavern with a roaring waterful, so communication between the PCs is impossible (dont let players talk to each other while in the cavern, etc).
Exposure to radiation/magic has altered the monster. Maybe now they can breathe fire (1d4 pts a round), have claws (extra 1d4 dmg), or possess a mental power (mind blast). Especially effective if the PCs are expecting "another group of goblins" and get waylaid!!!
Have the critters working for a "boss" monster that doesn't want to bother with battle, but will jump in if "his boys" are getting worked over. Suggestions could be a displacer beast, medusa, exiled drow warrior, or intelligent giant arachnid.
Give them potions! Great one use items you don't have to worry about falling into the hands of PCs should they defeat the critters. Good potions for use include Invisibility, Growth, Healing, Speed and Diminuation.
Maybe the critters have OTHER critters working for them! A pack of giant rats, wild dogs, or jermlaine (tiny evil creatures) could be serving the monsters as watchdogs.
Make one of the critters a shaman or mage, and give him a 2-3 charge wand of something like Paralyzation, Fear, or (mean nasty DM) Fireballs! Should make the PCs take those kobolds a lot more serious when a fireball explodes into the middle of the party!!!
Another technique: take a higher level humanoid monster, and rename it Kobold Whatever. If it has any traits or powers that are specific to its type, like the hobgoblin defensive abilities or the goblin ability to shift when missed by a melee attack, remove those. Then add the kobold Shifty ability. Bam, you have a higher level kobold.
Reskinning is a really useful 4e monster trick; once you identify the characteristic traits of a given type of monster, it's easy. See also the themes section of the DMG2 for more ideas of this type.
I can't comment on the 4e system itself but there are several ways of upping an oppenants threat. The least of which is to put a single more intelligent / powerful NPC in charge of them. Kobolds following the orders of a Mage or other smarter 'leader' can use better tactics and devices without ceasing to be Kobolds. Rather than just giving them more HP and more damage, change the way they actually challenge the players.
Look at the way your players handle the average encounter and give the Kobolds a way to negate thier most common tools. A well lit chamber to thwart sneak attacks, an anti mage shiled of some sort to thwart magic attacks. Or how about a new breed of Kobold descended from some ancient offshoot with altered abilities and powers? You can describe the Kobold physicality but use the template for soemthing else for the actual details.
Reskin an existing monster of the desired level. Alternatively, bump up the mook version of the creature so it's of the required strength. One of the neat things about D&D (any edition) is that it's pretty easy to scale up a monster; add some hit dice, perhaps give it an extra attack or two, and you're set.
Just remember to not overthink things. The simplest solutions are often the best.
I agree with cdtatro. Reskinning is the way to go. If you can justify tougher kobolds, storyline wise, then its easy to reskin most monsters to fit the bill. Reskinning lizardmen will take them up a few levels, or skulks... really, most martial humanoids work well for fodder. I'd avoid brutes and lean towards skimishers.
Also, don't discount terrain and tactically advantages. Kobolds are, after all, known for their trap making skills. Put the players in a hazard filled map, throw in a few brutes or soldiers of their level and surround them with some Kobolds throwing acid and alchemical fire (out of reach on a ledge). Half a dozen ongoing damage zones start adding up quick no matter what level the players are at.