The mission is seemingly impossible; guide your force of elves to fight their way past a large group of trolls taking their ease in a mountain pass, with precious little forest to provide haven. Fortunately, there are dwarves on the other side of the river, and while they are jerks who won't let you cross to avoid the trolls, they are sufficiently jerky to get pissed off at any troll who crosses the river and will charge across the map to kill the whole tribe, putting their sharp axes to good use and providing a welcome distraction.
Except, by some quirk of the AI pathfinding, not a single troll unit tried to take the shortcut through dwarf land.
Cue a slow grinding retreat back down the pass while those asshole dwarves get drunk off their asses. The dwarves do eventually enter the fray... with about four turns left in the mission, so there was no way to beat the time limit anyway. I didn't even last that long, sadly, as a plot-critical unit died in desperate defense of my home base.
Battle for Wesnoth: Legend of Wesmere campaign. Still doing the "Nuzlocke" permadeath campaign reselect thing, but it's frustrating to lose an early mission for such a stupid reason. I think what happened was I took a village on a little island too soon, so the trolls didn't clog the river trying to get to it, since that's how they usually stumble into dorf land. But yeah, no way to win that mission without getting dorf aid, unless you're lucky as hell sneaking the leader past the army (and are okay with something on the order of EVERYONE dying.)
Guess I'll see if I can get past the stupid stealth mission in the Liberty campaign now.