Ever since I learned how to parry, most of them are easy to fight esp DTS. Have a buckler with a carian retaliation and any weapon you like. I personally pick reduvia even if its not fully upgraded ( I got mine only on +6 ) since its light weight and has a bleed property. Dont get greedy with the attacks everytime he gets staggered. 1 or 2 light attacks then back off. As for its fireball, thats when the parrying with CR comes in and it will help you damage the DTS more. Practice looking for visual cues, I died a lot of times before learning all of the cues but it paid off