I laughed so hard when I saw this, that's exactly how I feel when I send an army into Moria and the Balrogs appear. My solution, as the high elves, has been archers... LOADS AND LOADS of archers. I break down the gate and then pepper the balrogs with 15+ units. I have to put one unit at the gate at a time so they have a sliver through with they can shoot (the balrogs always run up to the gate) otherwise they'll decimate my army.
As per Trolls. . I HATE TROLLS. My response to Olog-Hai are 19 shiny Eldarinwe Archers. Plant Stakes. Take Aim. KILL EM ALL. Sometimes I get lucky and take them out before they reach my lines (rarely) but by the time they do they're already so messed up by arrow fire that it's just clean up work then. Honestly, if it weren't for trolls I'd find this game to easy. They're the one unit that always makes me think twice. Playing as the High Elves with my Golden Army late game, the biggest challenge is facing of against the monstrosity that Mordor has become. The best comparison of a full stack of Eldarinwe Archers against a stack of ANY orc army is equivalent to a can of raid against an anthill, they die, ALL OF THEM. Trolls are the one thing that actually makes me pause, and Olog-Hai make me cringe. Those things do soooo much damage once they're in your ranks it's painful. But a nice line of Golden Deliciousness has always done wonders for me in the past