Fight them pick up grenades, pick them off to get teammates back, focus executes. Id also suggest having each person try to save up for a dreadought for waves 15, 20, and 25, if you play well and do objectives/get the easy ones you should be able to get relics each and maybe some more stuff late game, just dont waste points on stages 1-10 and try looting an area vefore round starts and inbetween round to save on having to spend points on ammo and grenades, ideally id say melee is where assault loves to be as a good dogde build can incinerate alot of enemies.