Multiplayer is laggy, and the random teammates aren't good. So in an attempt to beat it I used one of the dumbest and most broken strategy in the game. Charged railgun spam.
I built 2 of them, and used knight first skill, double damage condition, hard mode, and boss rush (to save time). Then I gave my teammate one of the railguns, and we spammed our way to victory. The third teammate didn't have a railgun though. I wish I had 3 weapons buff at the start.
Surprisingly, we actually struggled a bit at world 3. Even more surprisingly, the bosses didn't instantly die. It took something like 5 seconds. You would expect a total of 3 railguns (2 plus a dual weld) to melt bosses, but no.
At one point I actually got defeated. Luckily my teammates finished off the boss.
I would say that a huge part of the difficulty comes from the lag. Anyway, if you want to get the wizard skin desperately try this strategy. Charged railgun isn't as expensive as you might think. Also, boss rush along with challenge conditions also count towards this achievement.
The wizard skin from the achievement is really cool. All of the attacks are spears.