@Nobodave - the enemies shouldn't be invisible in this quest, they should just be normal Ogres. If they are invisible for you then that is through some sort of glitch. Re-validating your game files might help with this issue. We've had a few reports of invisible Ogres for a few players and are looking into this.
@pisir - Orcs like to fight, so there is no stopping the Tirranek from joining in. Take as many units as you can. I recommend using several battalions of Impalers supported by Marauders and some goblins (for healing). Send your Impalers ahead to intercept the Ogres while the Marauders fill them with arrows. Use your Hero as well to take out the Ogres and heal units after each ambush.