if( pc_isfalcon(sd) && sd->weapontype == W_BOW && sc && sc->data[SC_SOULLINK] && sc->data[SC_SOULLINK]->val2 == SL_HUNTER && pc->checkskill(sd,SN_FALCONASSAULT) > 0 && rand()%100 <= 15 )
skill->castend_damage_id(src,bl,SN_FALCONASSAULT,1,tick,SD_LEVEL);
This will give you 15% chance.
Edit the <= 15 ) to chance you want.