Keyblade, I think the Drill one is the only one where it's mandatory. All the others I know it's possible without.
Sonic Colors DS was actually a pretty good game, despite being a relatively new title...meh.
Ah well, I can tell you what happens, then - if you kill boss 6 with Void damage, instead of the boss blowing up as normal, instead it gets sucked completely into Void (leaving no trace), upon which Sonic reverts back to normal and you hear the explosion sound effect.