One thing you can try is taking him down with FEAR
It works pretty well. You just have to find a way of fearing him and he will become weak!
Easy to grab and easy to kill!
For the strat I would first check if he has any Vulnerabilities:
Vulnerable to Stealth Finishers - Can be killed instantly using
Vulnerable to Combat Finishers - Can be killed instantly using
combat finishers; Vulnerable to Ranged Attacks - Can be killed by
a single charged headshot, fire arrow, or shadow strike;
Vulnerable to Mounted Beasts - Can be killed instantly by mounted
Clumsy - Can be grabbed without needing to lower his health
Branded (Once dominated) Can be grabbed without requiring low health
Then, after that I would go for your best bet of winning such a strong opponent! FEAR!
Fear of Caragors - Becomes terrified at the sight of Caragors;
Fear of Ghuls - Becomes terrified at the sight of Ghuls;
Fear of Graugs - Becomes terrified at the sight of Graugs;
Fear of Burning - Becomes terrified when burned;
Fear of Morgai Flies - Becomes terrified when exposed to Morgai Flies;
Fear of his Rival - Becomes terrified when near a potential rival;
Fear of Betrayal - Becomes terrified when Uruks fight together (Branded vs. Free);
Fear of the Gravewalker - Becomes terrified at the sight of Talion;
Devise your strategy with these things in mind and you will surely kill him.
Well I think the skill you're looking for is
Summoner - Calls out nearby allies in a fight.
Basically he will always call allies to the fight so your best bet is using AOE attacks.
One really good at high levels is Wraith Burn.
So to sum all this up, you can stop him by killing him (dominating him) or running away (not that I think you want to do that anyway). And deal with the crowds using the strats I stated above.