First of all you have a bounty, so he has to fight on your terms because he wants you dead.
Lets begin with this that you had to fight between the hours of 10-16 so you can sear up his ass.
You never tried to slow the Kotrag which is a ninja killer.
You never tried to dispel him? Instead you tried the useless flashlight that can be healed even with red potions. The dispel magic has to be done when he goes for the caltraps/nerve because this is the only ninja skill that Kotrag uses and lags him for two rounds.
Well deserved death, good luck next time.