I solved my problem
If anyone ever wants to make monsters fight amongst themselves, here is what you need in order to do so:
-open your server folder trunk/src/map/script.c and look for the monster function :
BUILDIN_FUNC(monster) {
look for this line :
int mobid = mob_once_spawn(sd, m, x, y, str, class_, 1, event, size, ai);
then replace it to these :
int mobid = mob_once_spawn(sd, m, x, y, str, class_, 1, event, size, ai);
script_pushint(st,mobid);
Now the monster command returns the GID of the monster created
-make two monster, one with an IA of 0 and the other with an IA of 1, and save their GID to use it later :
.gid1 = monster ("prontera",153,170,"poring",1002,1,"",0,0);
.gid2 = monster ("prontera",154,170,"drops",1113,1,"",0,1);
You can now use all of the GID based commands and make an epic fight between a poring and a drops