How do you make a character with custom projectiles? Or custom meele weapons? And the same for monsters. I managed to make an enemy shot (by copying the bert monster) but I cant make it shot a single shot, and I dont really understand what I'm doong

Here is a tutorial for making a custom monster: https://www.lexaloffle.com/bbs/?tid=1448

Its a little outdated but still useful

It helped me a little. But I can't figure out why my bullet wont move. When the monster attack, it just stays in the air?

And also, is there a way to make an enemy only taking damage from behind?

your bullet wont move probably because you didn't gave it speed on the emitter.

I dunno

Yeah. About that, when I click single, I can't change the magnitude. So what I'm doing now is using ring with 0 angle and 0 dangle?

magnitude is only for spread and ring, on single you need to give the speed on the valocity properties if the bullet moves foward you put 1 on y and check the rotate with actor option, serously is written over there.

under valocity the only options are DX, DY and DZ. Nothing happends if I change the value

of course it happens, its the speed of the bullet on each axis

unless you locked it