How to beat a good MMA striker?

Me and my friends have some amateur MMA fights. My good friend is a very good striker, so i counter him with submissions and the famous Muay Thai clinch. He does not know much groundwork, but I do. I watch the UFC a lot and have picked up some techniques. I have choked/ armbar'd/ guillotined him out, but he has 3 wins on me by TKO from strikes. I rarely lose cause of my groundwork. How do you counter a guy like this? Please help!