Finally found the HP de-buff reason. I was using Twinsage Glintstone Crown which gives Intelligence but depletes Stamina & HP.
Defeated Radahn in 3 tries, was fairly easy compared to other Boss fights. Thanks to the NPC summons.
Now in Lyndell Royal Capital.
If I focus on increasing the Dexterity attribute will that also increase the dodge distance ?
currently I am focusing on Strength + Endurance (so that I have more stamina and heavy load tolerance)
