Well followers level depends on what level you were when you got them, so if you got lydia at level 5, its time to ditch her. They must be really weak with crap armor for you to one hit them with aoe.
The DB initiates are invincible companions at the end of the DB questline. They don't die when on the floor like other companions.
My companions don't die on master difficulty with two hits from my 518 damage daedric swords, and I doubt your aoe damage is anywhere near that tbh. But I restarted the game because that was to easy, took the fun out of it.