In general, my philosophy (primarily for dragon/golem) is to survive the waves. Around stage 12/13 you need to stop trying to nuke your way through and treat it more like a CB fight. The battles are much slower but it is a strategy that allows you to build up from 13 to 20 as you improve gear & masteries. Speed is important, the dungeons are slow. If you get speed up close to 200 you will get 2 turns to the enemy 1, sometimes more. A speed aura (I use epic spider, high khatun would work as well) is great. In lieu of a speed aura, apothecary is a great lead for the defense aura plus he gives you the speed boost with his skills plus spot healing. Resistance is very underrated but will serve you well. Atk down (Spider/Coffinsmasher/Stag Knight) is important (need ACC 150+, probably 200+ at stage 20 -- I get resist sometimes with ACC 170-190). If you are lacking acc focus on buffs, def up / shield / heal /block remove debuff. Zelotah, Grizz Jarl, Doompriest, Maus Mage, Apo, Gorgo (obv revive helps too lol). A posioner (OB/Kael/Frozen Banshee) is great for dragon/golem boss u just need to get him there. Poison ticks will "skip" the IG boss vengance attack. That is, if the poison causes boss to cross the threshold INSTEAD of an attack then the minions never get revived. This is why Brawler (despite affinty disadvantage) plus 4 support can do Golem 20. And on dragon, poison hits hard. The dragon is slow so you should be getting 2 turns to 1 to land poisons, heal, shield, buff/debuff.
Spider: TM control, Dec Speed, Dec Def/Weaken; CC spiders w stun, AOE HP Burn
FK: Multi Hitter A1, CA or reflect damage, TM control, speed buff, speed debuff, heal reduction