Trolls are a powerfull unit on early-mid game, they can destroy buildings easily, even with the nerf, and also they can get quickly elite infantry, the best way is hidden them from the enemie, wait a few minits and when the enemie armie lack of pikes, attack with them. Is very easily to use them against rohan, and dwarves, because dwarves are very low in numbers at the start This make them vulnerable to monster attacks, even for a phalanx unit is very difficult to get down a troll, and rohan peasants die easily with 2 or 3 troll punches.
Troll early game strategy could be interesting, also remember while a troll is near , the enemie player will push him, allowing your heros to try get enemie heros while the troll is distracting. With a good combination you can get enough territory to start building an outpost, then their use is your choice, but with the outpost orcs you must be allowed to win the match.