I cannot kill this done of a dragon please help

Are you using Dragonrend to force him to land?

First encounter?

after getting him under 5% HP stop hitting him and wait some seconds.

Check the Wiki and UESP about bugs. I've run into issues where he won't take any damage even when rended (word?). I think loading a previous save helped.

Hide behind a pillar and let the other three kill him. Just shout as needed.

Easiest boss battle I've ever seen in my life.