Lure them into a boxed street then pepper them from above with arrows, bolts, whatever. Take out their eyes first, then you might be able to have the trolls attack each other accidentally.
In the non-PJ world I would just have a White Wizard at the door, and that would be the end of that (sigh). Or maybe just a little sunlight bounced down to the gate from some mirrors would turn the first trolls through the gate into stone, plugging it up.
In the PJ world one could:
- have Legolas crush them with falling oliphants (one dead Oli at the gate would have been a big help).
- drop flaming Stewards on them from the Citadel of Gondor ("Hi. You're the new Steward," <pour oil> <ignite> <jump>, "Next!") as the force of impact from the same has got to do some damage, even to Trolls. They would be like self-guiding 'smart' bombs, and the flames can't hurt either.
- have Arwen the She-Elf say something dire then flood them with Minas Tirith's sewage system.
- intentionally collapse a stone building on them.
One of the first three trolls through the door goes down, and so I would repeat the same action (can't remember what it was).