This is easier in TDM. Both assists and kills count as eliminations.
Each elimination in TDM counts for 40GP, meaning you only have to get 10 kills / assists to finish this quickly. I got this my first attempt.
If you decide you want to go the 2v2 Gnashers route and you want to get absolutely sweaty, eliminations are worth 60gp, so you’d only need 7. While going for the 10 wins achievement this should happen naturally (hopefully).
If you wish, you can go into KOTH to get this done, but it requires much more kills / assists at 27. You can get these both quite easily, but my advice is TDM.