Firstly this must be done in a public match
so either grab a friend/booster or a second controller and create a tumble mini game and set it public and snowballs.
Once in the lobby press
as soon as possible to ready up and start the match with just you. Have the person going for the achievement stand on the end of the last level of the map but facing inwards then have the person not going for the achievement stand in front of them.
Then the person going for the achievement will spam
as they walk backwards off the edge, might be an idea to aim higher too to avoid hitting the block below.
Heres a video of me doing it using 4 controllers, I'm top left (was going for something else at the same time)