If you get to The Void without this, once you get the sword that recharges your energy, wait for 6 pips to charge and do the big fights right after, just jump off after all but one monster is dead and you’ll respawn with all the fights ahead of you.
It’s possible like many of these ability achievements you don’t actually need to be in combat for it to count, but just in case you do.
Also, Flurries where only one monster dies still count.