There's 2 ways to crush any boss with the Duelist:
1- rely a bit on luck and use X Burn (max it as soon as you can) -> Summon Fire III or Field Volcano
It's a good idea to combine this with the Agent's TROY:Fire -> Hack -> lostpower.exe
2- rely on lots of luck and try to cast the 3 traps so you can use Judgement
The Agent is good for quicly wiping weak enemies and restoring the team's AP.
For boss fights it's mostly a support character. You HAVE to max Hack skill level and use TROY skills otherwise you won't be able to hack any bosses.
Some, like the Shadow Realm's boss, are especially resistant to debuffs so all you can do is try Knee Break to stun and at least make your team strike first.