Trolls come in 2 main varieties: With giant hammers or with grenade launchers.
Player 1 must destroy of all the troll's chest armor. The quickest way to do this is by jumping with and then performing a finisher ( and towards the enemy at the same time). This should take away all his chest armor.
Player 2 must go behind the troll and mount him by pressing . You must then balance on the troll by pressing the in the opposite direction that the troll is leaning. When he is upright, push up to perform the finishing attack.