I haven't been able to find any concrete information regarding this. I've been clearing the dynamis wave 1 bosses with just 4 characters recently to unlock some relic gear upgrade paths and have noticed that there is a damage taken mechanic on the wave 1 and wave 2 bosses. I'm not sure exactly how it works, but I've found that if I continue to use Trueflight that it gradually builds up a damage resistance to it. Trueflgiht damage goes from 30k to around 5k. The damage resistance is reduced and eventually removed if I start using Last Stand. I've found that swapping back and forth ends up with the fastest clear time with RNG RUNG BRD GEO. On the wave 2 boss, I end up using Coronach instead of Trueflight. Damage goes from 5~6k to 800~900 until I start using Trueflight (haven't experimented too much on the wave 2 boss though.)
Does anybody know exactly how this mechanic works? It might be as simple as magic versus physical, but it might be deeper than that. I end up doing skillchains when switching between Last Stand and Trueflight, so it could be related to that. I know that it applies to wave 1 and wave 2 bosses, I haven't tried on wave 3.