My build is https://grimcalc.com/build/1008-2GLSW8
Short story short, either with 2h slinger (61K DPS, 29% lifesteal) or 1h gun/shield oathbreaker set (46K DPS), fire/cold/lightning/vit/aether/chaos resistances maxed out, 25% stun res (could go to 65% stun res replacing head item), 7.5K health, i’m getting my ass kicked by The Sentinel.
I can’t kite him successfully, facetank is out of the question, he keeps summoning mobs/crystals and he is extremely fast, curse of frailty doesn’t work on him to make my life easier.
Ofc, trying to lure him out of the crystal, sometimes I die before even he summons the crystal.
I can write which equipment I use, although I guess at this point all necessary info is given already.
*Ultimately, I killed him by luring him with pet attack to the portcullis while I was behind it, he kept resetting his position (not his health though!) when he was reaching it but I kept firing at him before he reached the back room so he was going again to the portcullis. Rinse and repeat. Bugs rule! 
Don’t know for sure how he can go down with normal fight…