The simplest way to do this is by knocking back yourself. First, get on low hp (blast your triple RPG into the floor 3 times in a same spot (make sure you dont fall into water), then you should be on about 25 HP. Then blast the floor again, and when you're in midair, blast the floor again and fall to your death. You should have recived the splashback cashback achivement.

Btw this is my first guide so plz comment if there is anything wrong.