You can grab this achievement easily in the Helm's Deep mission do not destroy any of the ladders on the wall and switch to Legolas and constantly kill any Uruk-Hai that appear
you can also get That one counts as mine! at the same time.
Any easy way to farm this achievement if you have beat the game is to buy the Uruk-Hai character from Helm's Deep at night time on the over world no other name variation of the character will work and switch the second character to Legolas and constantly kill the Uruk-Hai.