I already tried shooting it but it doesn't budge, is something wrong with my game or are there other conditions I have to fulfil
nope there isn't, he's just hard to get so you gotta line him up perfectly with the shot so that it hits him I tried that myself and failed while others have succeeded. But there is a way to get past him without the gun, just keep running when you encounter him and don't be in the same path as him I noticed he can't stun you if you keep moving.