Hi! Can anyone explain me how to make double damage on boss 1 and 3? I don't get it. Thanks !

If you walk while carrying a box / 2nd player, after 30 frames of walking your projectile starts blinking, it means you have "charged" your weapon -> double damage.

Perfect distances for charging here:

In other situations you should go left and change your direction to the right in one frame. Frame perfect right to left is the same :) Good Luck

Chip `n Dale 2 Rescue Rangers 2 - Box Charging Guide