Try looking for some forum build guides and articles on recommended stats, how resistance reduction works, etc. Celestial content is hard and you have a ways to go.
Short version: You needs much, much better stats, like health, OA, DA, and absolutely MUST have maxed out resistances to all damage types except physical, and most need to be overcapped. (Your linked build does not have augments and such yet, which can help resistances a lot, but you might still need to swap items to enough.)
You should also ALWAYS take the devotion that reduces resistance to your damage type, in this case Solael’s Witchblade.