One Step From Eden

Play as any character except Reva. Fight Reva as soon as possible by selecting the Ruins area, then spare her when you win; her assist saves you from defeat once and throws out a 400 HP heal. Now on any boss after her get the boss within KO range and let yourself be defeated; when Reva saves you, dodge the heal and kill the boss. Achievement unlocked, and if you're fast enough you can even grab the healing before it disappears to help continue the run.