Gunmen naturally have a harder time with MKT's, but there are tactics to emply to give yourself a better fighting chance.
1. Adjust your base stats to give you more defense against MKT's.
2. Just because your a duelist, don't mean you can't use abilites from the other gunmen trees, mix it up
i.e Use clamors with SMG to get stuns on the MKT's, various roots from other trees to slow a MKT down.
3. Powerless is your friend, the gunmen tree has a move that has 50% chance to cause this, use as often as possible.
4. Don't try to go simply for states. You seem to be having problems with that, so use more moves that don't require states, pistol aerial, dual pistol execution, disarming shot, covering fire, etc etc.
5. Defense is your friend, learn to use grab and block to their potential, you will find yourself handling a MKt much better.
Just some basic stater tips, as anything, try a few things and see what suits you.
Fighting a MKT in pvp and a duel are 2 different things, so gain experience in both.