-When facing large groups of enemies, have a lot of defensive structures in your base and teleport the opponents to your base. The towers should kill the enemies before they can do significant damage. The Convert spell will also work as they are free men.
-Note: This strategy works best with elves.To kill armies (including brutal) easily, get as many Mirkwoods as possible. Have them guard your fortress while making sure they get Silverthorn Arrows. Then, get Glorfindel, Arwen, and three to five groups of horses. Upgrade them completely and send them around the back of the enemy base. Have them attack. This works almost every time. Make sure that Glorfindel and Arwen are on their horses. It also helps to have Rally Call and Heal.
-On the Minis Tirith, Minis Morgul, Helm's Deep, or Isengard maps, build your army and close the gate. Place three battalions of Archers on the walls of both sides of the gate. When the enemies start attack, kill them. You can open and close your gate quickly from time to time to taunt them. They will try to enter when you open the gate, but your archers should take care of them.