For fighting a Mumak, you will want him to trample you as little as possible, so heroes with a lot of might, and/or high defence, wounds, and fate. Boromir Captain of the White Tower with shield would be pretty good, with Prince Imrahil being a somewhat cheaper, but also a less reliable alternative. You have 6 might as Boromir, so you can call a lot of heroic moves if you lose priority, or to counter the Mumak's heroic moves if he does any. If he still manages to move before you by winning the roll, you will want to have Boromir placed so that the Mumak will hit him without first trampling too many of your warriors. If he is not using tusk weapons, he only gets 3 hits against Boromir, which he should be able to survive, forcing the Mumak to stop and not trample anyone. Boromir's defence 8 (with shield) will make the Mumak's trample only would on a 4+ instead of the 3+ of most of the other stuff. Even if it does inflict 3 wounds however, Boromir should be able to absorb that with his 3 wounds and 3 fate.
Once you get the Mumak to stand still, either by Heroic Movement, or by stepping in it's way with a hero, you now need to send as many men to fight him as possible. For this, Citadel or Fountain Guards are excellent, as their bodyguard rule allows them to charge the Mumak without taking courage tests. Dol Amroth knights are also better than most Gondor units as they have a courage of 4, and if you chose Prince Imrahil instead of Boromir, they can be pretty deadly against the rest of the Haradrim (since the Mumak is not the only thing in his army) because they count as being within range of a banner when Imrahil is within 28cm (14in?). Also, you will want people who hit a big harder as well. Clansmen of Lamedon are perfect, as they have 2 handed weapons, and their courage is 5, making it easy for them to charge the Mumak. If you take Angbor the Fearless, they will have the banner bonus from him, though at half he range, and Angbor also hits hard, having a 2 handed weapon, 2 attacks, and might to use if you need it to wound. You are relying on Boromir's fighting value which beats the Mumak, and the attacks of Boromir and the non-2h weapon warriors to win the fight, which should not be hard if you have a lot of men. Then, the guys with 2h weapons will be able to wound the Mumak on a 5+, while others will need a 6. You don't need to kill the Mumak, just wound it enough times for the Commander to fail a courage test (he takes this every time the Mumak is wounded) and it will stampede, allowing you to move it in the next movement phase instead of him. If you are near a board edge, you can move it off, or you can attempt to kill your opponents troops with it.
Another thing you can do before the Mumak even gets to you is to shoot at it. You can do this, and hope for a stampede, or just try to weaken it enough so that you CAN kill it later in close combat. If you do wish to try this, the Blackroot Vale archers are better, as they get some bonus when shooting monsters (don't know the rules, and have never used them). However, I would recommend taking Rangers instead, and not shooting at the Mumak, but trying to kill the commander at the top, making it far easier for the Mumak to fail his courage tests when he does get hit. The rangers also have higher courage, allowing them to charge the Mumak as well if need be, and higher fight (I think?) making them better against the Haradrim.
Also, use terrain to your advantage, hide behind big cliffs or whatever else that the Mumak can't pass through, and try to get the Mumak to get close to you without being able to charge (it can't turn during the movement phase, only the beginning, so if you are behind a huge cliff, it will have to come around one turn, and only being allowed to charge the next turn, before which you may have a chance to attack. Also try to keep your men spread out a little so that if he does get a successful charge in, he won't get all your men at once. This may be difficult while trying to put a hero in front of him at the same time, but you can try to see what the best option is during the fight.
Gondor and Harad are my two biggest armies, and though I have not played AGAINST a Mumak, I have used the Mumak a lot myself, and I know that Mumaks are very easy to kill with the right tactics. Just yesterday I got swarmed by Khazad Guard and Dwarf heroes who took my Mumak down in 2 turns of close combat, and all the crew fell to their death, and I only killed a shieldbearer and a few Khazads during a charge.
That's just my two cents.
Thanks!
Maermaethor.
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