Open up slayers and choose the map public assembly. You can boost this very easily by inviting a friend and getting them to idle however, it’s easily doable with a random person just keep your distance so they don’t kill you. There are clocks around the map you can break for more time if necessary.
The character you want to select is S.T.A.R.S Wesker. His handgun is around a two hit KO for normal enemies and you also have the L.Hawk Magnum and Hydra shotgun for emergency kills if you’re running out of time to extend your combo. You need to hit a combo of 20 and if too much time passes or you die the combo is interrupted. By killing an enemy you increase the combo by one therefore, wait for a cluster of enemies before starting the combo and run further into the map towards more enemies (If you have a friend they can round up majini and lead them towards you). Killing other players also adds to your combo but it’s easier to kill generic majini. (When your combo begins to flash/blink it’s about to end so kill something quickly)
Hopefully this guide helps and you find it easy enough to get the 20 combo! :]