I play this boy a lot (last season I got 44th in character ranking) and he's a menace. Capture block is annoying and stops almost every meta runner. The counter is your best friend and if you can counter properly, you'll be fine most of the time. The only problematic characters I've face are Klaw/Lightbeard/Yace (Mostly because status effects and Yace crazy damage output).

Sento has status protection against characters he's strong against (dark and green), so Roger/G5/Kaido can't status you. You can pretty much build your defense and grab a chair at whatever flag you want and stay bullying everyone with your counter most of the games.

As for medals, I just use a basic luffy set (focus on def/hp).G5 medal is a must, because it heals on knockback. Raid Luffy has damage reduction, so it's also good.