Trolls are excellent in numbers. As another poster stated, once one Troll is downed, it doesn't matter if you need 10 rounds to finally kill him... except if there's seven more Trolls coming for you!
I used an encounter with six standard Trolls plus one elite Troll (2 levels of Fighter, nice gear, good feat selection - tripper build). That encounter was only EL 11, but my three level 12 PCs were in serious trouble. The Druid was on cremation duty, but she always took longer than a round to finally kill an unconscious Troll. The melee guy was whacking away, and his good AC helped him out a lot, but he still took a few unlucky hits now and then. The Sorcerer confused two or three Trolls with illusions and Charm Monster+Diplomacy.
But when the first two downed Trolls got up again, the fight turned into a long, happy mess. The Druid had to help out in melee, the Sorcerer tried to CdG downed Trolls with Alchemist's Fire (which didn't work so well), the melee guy had to be in-combat-healed repeatedly. It took a looong time to finally really, really kill all those Trolls.
Another sweet idea I had was using Trolls as living bombs. Why should Trolls be afraid to jump down a cliff if they can hit some unlucky adventurers at its bottom? Spot check, then Reflex save to avoid a nasty 20d6 "falling Troll damage". The Trolls? Got back up later!