To defeat Maero the first time 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, just shoot 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 ammunition to shoot his Monster Truck.