This one got me a few times too... I assume you're talking about the encounter troll that spawns in the entry area where the last battle takes place?
What you need to do is take him out before talking to Stothkell and initiating the last battle. If he spawns, walk right by Stothkell and go in and take him down, then go back to Stothkell and start the last battle. It should go reasonably easily then. (Sounds like you may have tried this though...) What kind of skirmish soldier do you have? A healer is very helpful here.
Worst comes to worst, you could always do the skirmish again because since he's an encounter, he doesn't always spawn.
Galadhloth, Hunter 100, Glirithil, Mini 100, Calanor, LM 100, plus lots and lots of alts
Watchers of Elendil ~ Landroval ~ Crickhollow ~ Tarciryan Knights
~~~~ Imladris in the Forgotten West ~~~~