"It's mainly about getting as many kills as you can, and to keep the same pattern for each map. Also the less u die the more ur skills increase and the opposite if u die. healing others as well does increase ur skills slightly. Lastly be careful with TK'ing other players and dying cuz these are the main factors of losing skills." Espe commented this on my question similar to yours, as I can't give you my own advice because you saw how bad I play
Skill= (bosses+defibs+(healother-heal))/playtime. It was mentioned way back then. Not sure if it's the same