To defeat Gorthmog the Orc Army Commander, you should have these abilities (or better) and items: Morgul Decay, Dispel, Gift of Galadriel, Citidel Rage/Ecthelion Wrath, crippling moves, and Arnor Mushrooms.

First, use Morgul Decay on Gorthmog. Now his armor is zero. If you kill one of Gorthmog's minions, he uses Black Gate to bring them back. Use crippling moves to keep them out of battle. If Gorthmog's minions use Mordor Mirror, use Dispel to get rid of it. Gift of Galadriel will help with healing Faramir, since he has powerful attacks and tons of health. Use Arnor Mushrooms on Berethor if he has Citidel Rage or Ecthelion Wrath so his moves are powerful. After Gorthmog goes chicken and runs away, finish off his followers and then you win the battle! (Perfect Mode really helps in the battle.)