Did you not notice the elves are struggling by the time they get to the frozen fortress, they are exhausted, also in a frozen climate that would also effect them and trolls are not an easy foe, do you expect them to be at 100% effectiveness at all times just because they are elves, they are sent out to scout for the last remnants of sauron not fight it all themselves, the elves just got knocked down noone was injured or killed, legolas was shown to one shot an elephant with a few arrows that couldnt possibly reach its brain even at point blank range, not everything is supposed to make sense.