This can be done with the jungle camp monsters, I got mine on the Mana Camp on Joust.
Hit them once and then retreat to the very edge of the boundary. When you're below 10% health, activate your Ultimate and then step out of the ring. The monsters will retreat and when your Ultimate wears off after the four seconds your achievement should pop.
Please comment if this get fixed in a future update and I'll delete this solution, as I know Hi-Rez can be quite crafty with fixing stuff like this. For now though I can confirm it works as a group of us all got it this way in a session.