The easiest way would be using DK's plug-in which supports frame by frame animated bust and very easy to use.
We used dragonbone support plugin and some custom eventing to make even smoother bone/mesh animation.
Hope this helps :)
@alice_gristle Haha yeah! This is one of the rarest ultimate attacks so we indulged ourselves and made it super flashy with a long sequence. There will be shorter and less powerful Overflow mode attacks in the game.