Something else you can try is to pair as many pest damage debuffers as you can with Kerria once you get her to level 80 and equipped with Taunt. Forcing pests to overcome 80% dodge for two rounds followed by 50% for the rest of a fight, combined with up to a 70% reduction to their attack power, can allow Kerria to survive forever (barring bad RNG regarding skill activations) while the rest of the squad helps wipe out the pests.

Even the 'normal' raid boss in each stage won't be able to do much if you have a strong enough attack debuff on it.