This is not a hard achievement to get at all.
His ultimate is Mystic Rush. He leaps to a target location(in range), and deals damage to people in that area, and it is not hard to successfully do at all. The assists, and kills you make both count towards this achievement, so all you have to do is just put damage on enemy using this. The kills do carry other from game to game, so while you work on the other Ravana achievement you will probably get it.
Arena is the gamemode I would recommend because it is all team fights, and you can jump on 2-3 people at once if you are lucky. I was able to get the achievement on my second game, so this is one of those achievements you don't really need to hunt after because you will get it through mastering the champion.