I think it would be a good idea to reflect, in the weather spell (because, let's face it, every faction has had some kind of weather spell for tier 3 and I don't think the Misty Mountains will be left out), that the orcs, goblins and trolls of the Misty Mountains faction are not the same kind as those of Mordor and/or Isengard.
Here is the lore-reason: Saruman bred the Uruk-hai to be faster and stronger orcs that could endure the daylight (regular orcs were weakened by it, which is why Sauron sent the Dawnless Day ahead of his armies before the Siege of Minas Tirith). As we know from The Hobbit (the book, at least), trolls turn to stone in daylight, which is why Sauron bred the Olog-hai to be smarter and stronger trolls that could endure the daylight.
Now obviously it would be difficult to have all the Misty Mountains trolls turn to stone once they're trained and only be available under MM's tier 3 weather spell (to say nothing of unbalanced), so here is my proposed solution.
Misty Mountains' Tier 3 weather spell will be something that affects the weather, maybe a swarm of bats will cover the sun or something, and it makes the goblin and troll units move faster. I think something of this sort was mentioned on another thread, but I can't recall where.