I have about 1800 inning in my progressive and am really unsure of how to use Kris Medlen.

Pitching 138 innings, he appeared in 50 games for the Braves and started 12. He was lights out.

Do I start him, use him in long relief, or make him the closer?

What do you all think?

His IP/G is 2.76 - so he definitely can't start, but with 138 IP and a 0.91 WHIP I would set his pitch count  to go 2 innings a game and make him the closer. 

Tandem B. 40 pitches every third game or so, lower your SP's pitch count for that game, or pair Medlen with a low IP starter.

Could also make him your only Long A and bump up your starters' pull settings so he's used more often. If you have a decent offense, having him keep games close before a SP can get knocked around might be worth as many wins as having him close, and you'd be more certain to maximize his innings. That assumes that some of those 1800 innings are other good closers. If not, what italyprof said.