You don't mention how far you are currently with the dungeons, I'm guessing something 12-14 like on most, bit less for FK and 8-10 for spider?
For FK, there are three main criteria (besides simply enough survivability and ability to clear the waves): multiple hit attacks to take down the shield, heal reduction and turn meter reduction to keep him from healing back up and delaying the return of the shield. Lightsworn will help, you could also consider developing Athel or Coffin Smasher - the latter is also very good for Clan Boss. Armiger (yes, the uncommon) could possibly help with turn meter reduction - also on spider. Juliana maybe, with a 2-hit A1 and poison/hp burn on the others. You lack good heal reduction options though - only an unreliable one on Kallia's A2 which probably isn't enough reason to bring her.
For spider, at least at low-mid levels, it's all about AOEs on the spiderlings and if possible turn meter reduction on the spider. Bellower would be good here since all his attacks are AOEs. Kael and Warmaiden obviously, HK for aura, maybe Dark Elhain or Athel.
For dragon you should have better champions - poison works well there, so I'd suggest FB or Juliana in addition to Kael, Warmaiden, HK and somebody for support - not sure if Roshcard, Mistress of Hymns, Spirithost or Doompriest is better for that, they all have their advantages.
Royal Guard will be great on higher stages because of his attack based on enemy max HP, but probably won't do you much good yet now.
