What is the worst boss fight, and why is it Redahn?

Going on 3 days stuck on this guy. Most gimmicky fight, easily 1 shot killed, get knocked off mount and trapped in combos. Summoning players are basically useless here, npc summons work as a distraction and that's about it.

I've tried using blood loss and got damn close , tried blood loss arrows staying a distance and still no good.

Cheers to all of you who has slapped him around, I'll get there eventually!