Originally Posted by mcnichoj
Certain synergy attacks have certain critera that must be met before they active like Bubbles right?
They shouldn't. Just pick up the assist item, and if it's your character's synergy partner, it'll happen. Although it seems like certain stages don't allow synergy attacks. The Foster's foyer stage, for example, wouldn't allow them, but the KND treehouse and C.O.O.L.B.U.S. do. Or maybe it's just the game being buggy.
I, for one, did all the synergy achievements in the last Samurai Jack stage in story mode (the bridge).