In this case. It will be Halo Legends Arbiter, aka Fal 'Chavamee which was one of the most powerful Arbiters ever and the responsible for The Arbiter bad reputation.
So here's the deal. Deathstroke is in Shanghelli to see some 7 feet tall chicks and see The Arbiter wife. He bangs her and kills her after, just like in Halo Legends (except for the bang part). The Arbiter discovers and goes on a rampage to find Deathstroke. And he finds it.
Deathstroke is able to summon 10 minions. But if he do that, The Arbiter will be able to summon 2 Field Marshal B.O.B.S. Which are the hardest enemies to kill in Halo.