The Soul Arbiter isn't as challenging a foe as it may seem after surviving all ten levels of his maze. It doesn't mean, though, that you shouldn't do him justice - he's very agile and can deal pretty lot of damage. One of his primary attacks includes a series of rapid strikes.
The second of his favorite attacks involves sending out a shockwave, which is caused after he jumps into the air and crashes down with mighty force. When that happens, try to maintain a safe distance - the shockwave can take a substantial amount of your health.
I don't recommend using Death Grip in the battle with the Soul Arbiter; it won't do too much good here. Instead, use standard attacks, such as Harvester Revenge. It also pays off to apply special Wrath attacks, such as Harvest.
When the Soul Arbiter looses around 1/5 of his health, he will surround himself with a protective barrier and summon Skeletal Archers for aid. DON'T attack the boss then (he'll also just observe your actions), but focus solely on the summoned creatures. Resume the offensive against the Soul Arbiter only when the barrier dissolves. The same thing will happen later on (with Skeletal Champions to kill along with the Archers) - again, stop attacking the boss until all of his helpers are down.
Another surprise waits after the boss loses around 2/3 of his health - he'll summon an Undead General, BUT despite the protective barrier he'll still keep attacking you. Unfortunately, you won't be able to respond to his offensive, so until the General is dead, dodge however you can. This would be a good time to use the Reaper Form, or any other special ability that will allow you to quickly dispose of the General.
Continue hitting the Soul Arbiter until you empty his health bar, which will ensure your victory and reward you with two legendary items - a secondary weapon Executioner's Hooks and the Crown of the Dead talisman. Don't' forget to pick up any random loot too.