Ruby Void, Amber Void, Emerald Void can all be easily completed by that specific color of drake simply not being alive at the END of the fight. You can get 2 Void achievements by bringing the boss to low HP and then dismounting all drakes but 1 to finish him off. I believe you can get all THREE void achievements if EVERYONE dismounts and you finish him on foot, but I have the achievements and cannot test this.