I have no clue how I died to him that first time but all you need to do is stun lock him because he deals A LOT of damage with one hit. My health here is close to 600 and an armor value of ~1700

Dragon Aspect + Secret of Strength + Elemental Fury + Enchanted/Tempered Armor + Maxed Skills and Perks makes this an easy fight on Legendary Difficulty. Especially with the perk that allows power attacks to stun.

Also, no alchemy exploits were used. Just store-bought potions of enchanting (20%) and smithing (50%), also some enchanted armor pieces with 28% Smithing each (Gloves, Armor, Ring, and Amulet)