When you are up against a troll, you can sit behind him all day and melee him. It is a matter of timing. With the brown trolls, you wait until they lift their arms up to knock you. hen they do so, dive (y Button) behind them just before they do so and continue to melee them. Repeat as necassary.

With the Gray Rock Trolls, you melee until they are just about to jump in the air, then you dive away from them. Once the rocks they thrust out of the ground go away, move right back up to him and continue to melee him. Repeast as necassary.