The Warlord Gathrik is one of those baddies that levels with you. At level 25 you have jst came into a new tier of baddies. As a two handed warrior, you don't have the ability to block and actually stop a lot of damage. Which means you need to move around and dodge his attacks. Power attack, then regular attack then get out of the way. Also I don't know if yo have any shouts at your disposal but Disarm would be a good one. He deals much less damage with his fist. Fire Breath wold do a good bit o damage as well, but when in dobt Fos Ro Dah!