This requires some finesse and a character which can upgrade Speed doesn't hurt. You need to change team to Zombies, pick a character like the Imp who can apply Speed.
Now you just need to keep count and survive 25 'rounds'. The first 5 are simple, but every few rounds it increases how many squares are INSTA-KILL. This is not bad if you just stand near where 4 corners meet and be VERY accurate with where you place yourself. It is easy to get killed even though you were already moved.
NOTE: I could NOT get this in Public servers right now. When I switched to private (due to inactvitity), I was getting my coin bonus, XP bonus, and it was counting the rounds correctly. As of Sept 9, 2019, do this in PRIVATE.