How do I deal with an overpowered Maniac?

You know how it goes.

There's this captain with a useful skillset, but he's got that annoying Iron Will trait, or he's just a level or two above what you can recruit, so you subdue him and use the Worse Than Death shaming to break his will.

... Except it goes wrong, and he becomes deranged. He's now a level 65 Maniac (I'm level 50 at this point). Between his 15-level lead on me, his traits (which include Berserker, Beast Slayer, Fire and Curse Proof) and the poison weapon he wields that one-shots me if he gets a single hit in, I'm barely able to kill him if I'm really careful and really lucky... But the [expletive deleted] also blundered his way into the Death Defying trait which means he gets a free full-health revive once every confrontation. And because he always chooses "humiliation" when he takes me down, I don't.

Worse and worse, he's an Ambusher, and he has already followed me across province boundaries twice, and he can and will show up while I'm dealing with other targets.

I can't realistically beat him as I am now. I can't make meaningful progress in the game while he's alive because he'll show up and disrupt whatever I'm trying to do.

This effectively stops the game being fun for me.

What do I do?

EDIT: Here's his specifics:

Ashgarn the Unashamed: Poisonous Marauder Tank (LEGENDARY)

Strengths:

• Immunities: Fire, poison
• Hates: Everything, Stealth
• Class traits: Tank, Determination, Wave of Might, Hail of Bolts, Front Line Warrior
• Bonuses:
  • Epic Traits: Mighty Warriors, Gang of Elite Warriors
  • Poison Weapon, Beast Slayer

Weaknesses:

• Vulnerabilities: Beast Fodder (useless, because Beast Slayer)
• Quivering (dazed by enraged enemies)
• Hints: Damage by ranged, execution, stealth.

Solution 1:

Gang up on him. Oh, and use a bodyguard.

Focus on his Quivering. Bring a Captain, or more, that is Enraged by poison. More damage for your team & Dazed on his part.

Don't give him an excuse to go Berserk. That's his biggest strength. Choose your gear as carefully as your allies.

Go to the army screen, find, from among your supporters, a captain that is immune to poison, and command him to kill the offending Maniac. You can command up to 2 others to support your Captain. Again, Poison Immunity may be important. Now you have a mission available on the map where your 2 or 3 orcs will support you in your desire to take down this trouble maker. 3 to 4 if you summon your bodyguard into the mission. With this level of support you should be easily capable of taking out the enemy orc's followers while your mob works on the Captain. Feel free to support your Captains as you feel is safe.

Because of the 15 level difference you'll need to use Worse Than Death and a 2nd mission just to reduce his level enough to make him recruitable. Alternatively you could just kill him for some sweet (L65 Legendary) gear.

As a bonus, the Captain you give the "kill" order to will gain some levels.

• If he is susceptible to poison himself, equipping "Dark" clan legendary gear should help. Immune, Enraged.
• Enter tough fights full up on Wrath. Elven Rage and it's free executions/ammo have helped me through many a tough fight. After you fire off your Elven Rage, Change into (some) Bright Lord gear to accelerate Wrath Recovery. (Avoid pieces that deal frost [elemental]) Perhaps change back out before using it again, so that you get full duration.
• The mission will last only as long as the captain with the "kill" order and the victim survive, the reinforcements can easily vanish when your point man dies. You may wish you give point to a "Tank" or a "Defender" or better yet a defender/tank.
• Berserkers become "Enraged" by a lot, making them extra dangerous, and the likely source of your "one-shots". Try to avoid his triggers.
  • The "Waters of Lorien" upgrade to "Shadow Strider" is a good way to avoid "Acrobatics". Not perfect, but good.
  • If he's enraged by caragors, bringing a caragor rider or beastmaster to the party is just asking for trouble.
  • Please tell me he's not enraged by spiders... (Enraged by Everything! /facepalm You killed your two Captains.)
  • All forms of elemental damage can provoke Rage.
  • All beasts of Mordor can provoke Rage. Bring only Orcs and Ologs.
  • Shadow strike and variants... Avoid.
  • Stealth...
  • Execution...
  • Don't bring along any Undead/Necromancers. The end of the Act 4 variant of Elven Rage, could be trouble. Be prepared. On the bright side, no point avoiding chain executions...
• Commanders can be nice as reinforcement as standard bearers are trouble for the opposition.
• Bonus points if can field a Captain that is both Immune to poison and enraged by it. Give him a taste of his own medicine.