Well in all honesty the only strategy that ever worked for me involved at least 3 players to get past level 40. The strategy mainly involves getting a good box weapon and using a wall weapon. Generally you never want to buy the block up the stair case but buy the one in the room with the box. If you have 4 players, have 2 of them up top and 2 on the bottom. Personally i choose the top so i can get the sniper rifle and the bar and keep using those two until i have enough point to spam the box. Run around as much as you can and reload every second you get a chance to.