I'd recommend doing the "fake" multiplayer glitch for this trophy, which you can find how to do here: https://pastebin.com/Jgf6vPeJ
Before starting the round, make sure you vote for the Snowball version of Tumble in the vote menu. Alternatively, when you're creating the round, go into the Settings and set the game type to Snowball only.
This one can be tricky to pull off, even in a solo game with you and the "fake" player two, as you have to precisely aim your snowball to hit them while falling.
Get the second player onto the bottom-most layer, and then follow them down there. Then, get them right up to the edge (but don't knock them off). Once they're at the edge, position yourself next to them, and keep your crosshair focused on them. Then, jump into lava and keep spamming L2 to throw as many snowballs in their direction as possible. If you keep your crosshair roughly aimed at them, a snowball should hit them and the trophy will pop.