In order to get his achievement from the start of the Helms Deep mission destroy the first three ladders so that the full length of the wall becomes available.
Once this is done stay as legolas and constantly run up and down killing all mobs allowing the Gimli to attack a few and the achievement will pop in no time.
(If this does not work try switching to Gimli and attacking an enemy once or twice and switching back to Legolas to finish them off).