First of all, bring a slash weapon as it is resistant to both blunt and pierce. Sword is best but axe will do if you don't have one.
Stand right in front of its center and react to his attacks like this:
-stomp with its left leg- this attack will miss you so just get some free hits.
-stomp with the middle- parry it then retaliate with sword secondary attack (or jsut hit him with axe if you don't have a sword)
-swing with its right leg - parry and attack just like the middle stomp.
Alternatively you can kill it with fire arrows but it will take a while. You can abuse its crappy pathfinding.