Phariseer is a relatively easy boss to defeat, though his strikes are quite fast. It's crucial to dodge whenever he performs them, and not too late. The boss has a few type of attacks up his sleeve, but they don't differ much.
When it comes to attacking him, I recommend the same technique as with the previous bosses in this mission, which is using Death Grip to pull yourself to him and perform a quick series of hit. Remember not to attack too much, though.
Each time Phariseer is dealt enough damage, he'll summon some lesser monsters for help. Initially, it will only be Skeletons, but in the later stages you'll also have Skeletal Warriors and Skeletal Archers on your head. To get rid of them, I suggest using area-effect attacks. Make sure you've killed all of Phariseer's helpers before you get back to killing him.
Since this is the main boss of this quest, consider using the Reaper Form, if of course you have it. Whatever tactic you apply, attack Phariseer until you empty his whole health bar. As a reward for defeating him he'll not only promise his obedience towards the Lord of Bones, but from now on you will be able to summon him as part of the previously acquired Interdiction ability. The game will also unlock a new main quest called [Judicator].
[TIP: Interdiction is activated by pressing the middle mouse button, but only when it's possible. The easiest way to use it is to select it from the roll-down menu which appears after pressing TAB. Interdiction is mostly used in solving puzzles, though as the game suggests the Dead Lords can also come in handy during battles. At the moment, you've only enslaved Phariseer, so he's the only one you can currently summon (naturally, it will change in the course of the game)].
[TIP: Theoretically, you could proceed to the next main quest by selecting another location from the world map. Nevertheless, I strongly advise reading the Closing the quest section to learn how to use the first enslaved Dead Lord to access a previously inaccessible area of Phariseer's Tomb.]