You should look at that video again, Combofiend does a very fast reset in the middle of the combo with the Shotgun.There's a video of Combofiend doing like 70 percent on Arthur solo with Dante.
Highest damage combo I have seen is with Thanos, around 60-70%. Thanos seems on the higher end of damage based on his normals but yeah, damage is not going to be an issue in this game.