To defeat Maero the first time while on top of his hideout, hit him once with an RPG. When scoring a direct hit, he will go on one knee and stay down. You can then shoot him until he is defeated. If he gets up, simplyshoot him again. To finish off Maero in the arena, use some of the rockets to blow up all the trucks except for his. Use extra rifle ammo to shoot his Monster Truck.