User blog comment:Victory awaits/Can witch overpower an Archangel?/@comment-9249716-20160126222752

Don Stark wasn't able to actually kill the leviathan he had cast the spell on, only render it stunned and not at full strength. Given an archangel's raw power, it is very doubtful that any magician on the show could actually kill one. Rowena wasn't even able to kill Castiel even with her power and experience.