One of the challenges whenever I run a large Troll unit is to figure out what enemy unit is running the Banner of Eternal Flame, because you do know it's going to be there somewhere since everyone fears the Hydra and the Abomination. A good guess is usually the heavy cavalry, since they usually hit with high strength at a decent initiative, and has the armor to survive the monster's return strikes. If they haven't brought cavalry, look for the medium sized unit with high strength attacks. This is usually very difficult for the Trolls to handle, since they have low weapon skill and extremely low initiative, so unless they have regeneration, they're going to take a lot of damage.
A good idea to protect your trolls from this is to buy a cheap Goblin Big Boss and give him a great weapon. You usually see a lot of these leading gobbo units, since they're so cheap and do pack a punch. But in order to protect the trolls, you also equip him with a magic item called the Dragonbane Gem. It's a minimum cost item, so you'll still have your goblin boss coming in at less than 40 points, but it gives him a 2+ ward save against flaming attacks. If you do suspect the unit threatening your trolls to be carrying flaming attacks, simply move your big boss out of his night goblin unit to stand in the way of the flamers, angled in a way so that they won't overrun into the trolls if he does die.
Oh I know he'll struggle to win battles even if he doesn't take a wound, but if the Warboss is nearby you shouldn't be losing combats by too much, so chances are you could have said unit tied up for quite some time, allowing your trolls to do their stuff unhindered by flaming attacks. And if you're wrong, and the unit you're fighting doesn't have flaming attacks, well it was just a goblin anyway.