I´m replaying this game and last time I remember that when I got the electric guitar I made a combo where Hector played the guitar and then bats appeared around him with lightning, but now I just can't figure out how to do it, can somebody help me?

Considering the only combo is pressing the square button a number of times before finishing the combo by pressing circle, I'd suggest you just keep trying the different attacks.

I'm pretty sure circle is the finishing attack. If you press it you have to start a new combo.