You’ll have to level up your character at the yellow statue until you get the move Lethal Flurry. Now the easiest place to perform back to back lethal flurries is in the final level, the Void. You’ll be given the Kladen sword to fight with and after fighting about 5 bad guys the next area you go to spawns a ton of enemies. Keep attacking and dodging until you fill up all 6 energy orbs. Hold down Y to perform your first Lethal Flurry, then just hold down Y again to perform the second. If you mess up just reload the checkpoint. Also a great area to farm the 20 Lethal Flurries for the other achievement.