This is the final boss of the main storyline and cannot be missed.
Credit for strategy goes to steam user rcole_sooner with minor editing by me: "Shock, shock, and more shock.
Be the correct level, get good shock weapons and grenades.
His 1st phase takes getting his shields down 3 times, then finally his health will be a target. Hit him with shock, hide behind the pillars, dodge novas, and shoot Sera Guardians for 2nd winds.
Don't shoot him while he is twirling his staff, or your bullets will ricochet and hit you. Also don't fire when he rises up as he seems immune and it just wastes ammo.
Jump to dodge his novas between shields being depleted. He jumps and slams his staff to create the novas. Just time your jumps.
Once the shields are stripped 3 times, you can drain his health pretty quickly.
Really the 1st phase seems the hardest.
The 2nd phase (the big empyrean sentinel) is 3 faces. The active face is what you need to be shooting. The middle face shield is resistant to shock, but the other 2 are weak to shock. After the shields are off the actual face is weak to fire, but really goes down easy with any weapon.
During the 3 face 2nd phase fight .... Stand under his chin and shoot his face. Jump on the floaty platforms to avoid the floor DOT phases. Butt slam when thrown up to avoid novas from his face. Go around back to heal when needed. Rinse repeat. Hope for 2nd winds from Sera Guardians.
I hate the last corrosive dot phase, but usually am fight for your life by that time and the Sentinel goes down in time to get a 2nd wind off of him."