If you don't know what the "???" boss is and how to get to it, read another guide. I'm only here to talk about one trick that makes the fight really easy.

To get the "friend" achievement, you must beat the fight without taking damage.

The first thing you're going to do is select a character. This is the entire secret to winning the fight: choose mebo. Why? Well, mebo can propagate. And if one of your mebos gets hit, it gets smaller. You won't take damage if there is at least one full-size mebo left. By letting go of LMB, the full-size mebo can regenerate the other mebos by touching for a moment.

Before you go fight the boss, focus on collecting "+1 mebo" upgrades. Of course, don't forget about other useful upgrades, such as speed or regeneration speed. When you're ready, initiate the fight. It's recommended to also use the 0.5 game scale trick. Notice how every single one of your mebos has its own orb. When you break the window, you might also notice that all of your additional mebos from the propagate perk stay, giving you twice as many mebos to work with.

Even if you get hit during the fight, concentrate your mebos together to regenerate. You will only take damage if all of your mebos get hit at the same time. Also, the orb power of all of your mebos combined is more than enough, so don't hold it for too long or you might fling the window to the other side of the desktop. The first time I tried this strategy after not playing for a few months, I got the achievement first try.