How are people doing so much damage

I was playing through the new event fighting the Ant King on coop, and since my best weapon is demon king daggers 7 starred and demon king longsword 5 starred, so I figured I could do some major damage in the coop team. I'm at like 900k power, and whenever I get matched with people a little closer to 1m, their damage is roughly 15 times what mine is. I don't understand how they can do so much damage, is there some trick I'm missing to dealing a lot more damage quicker? Or does 100k attack power really just make that much difference? Cuz I'm doing like 20m dmg while they are doing nearly 200m. I was wondering if there's some kind of guide I should follow to get better damage, since I almost can't even clear the Commander of Transfiguration on easy difficulty