This is just 1vs1 - no partner. He is by FAR, the hardest person I have EVER dueled in the entire game. I've tried every suggestion here, and only managed to beat him once on pure luck, our fights usually end with him at 50-90% health.
Anything I can do to record our fights, I want you to see it for yourself.
I can't slow him - he has a talent for that. I can't stun him, he knows the proper timing to use his shield (or has trinket up) I can't out run him - he'll steal freedom grip me back. The pressure he puts on is unbelievable, it puts me on the defense within X seconds of the fight. I can't get close, I can't run - I really don't know what to do.
You need to see it for yourself.
Your problem is you're using burden of guilt, not using glyph of turn evil, and you're using execution sentence. The 30 second cooldown stun will definitely help you get more up-time on him, and we won't able to stop it as much. Using glyph of turn evil will give you an instant stun for eight seconds when he uses lichborne to heal which will allow you to heal as well, and holy prism will give you more damage and healing than execution sentence.
If you're going to burst on him something I've found useful was seal of insight, it helps eat through his necrotics and while bursting you will probably find yourself actually being healed instead of it being absorbed.
I recommend that you get an addon to tell you what moves he uses, GladiatorlosSA (or something like that) is an excellent addon as it tells you what cooldowns, and trinkets they've used through an audio voice system. Alternatively, get deadly PvP mods. it's a text display addon to tell you what cooldown he uses. Now that you have either one of these you should be able to time your stuff right.
And about the talent you mentioned, most dk's don't used that talent, and it's a 30 second cooldown that lasts for 6 seconds so I don't see why you can't slow him? He can only steal freedom once every minute and within a minute you should be bale to freedom at least four times with clemency. And besides, you also have cleanse and emancipate to get rid of his slows. And since you can do all that you could switch to seal of justice and kite him and if you get to judgment him three times you get an instant heal flash of light, a 3 point WoG and you could use holy prism on yourself. For me that's 100k+ heal without necrotics on being stacked on you.
Wait until he's used anti magic shell to burst on him, and do not stun him during icebound fortitude. Use blinding light to stun him and heal up which may force him to trinket, he may have more than one depending on what he speced into at his tier 6 talents. If he uses remorseless winter get the heck out of there to avoid being stunned. You can use Divine Protection against his frost strikes, so when you see he has a near full bar of runic power use it, if you want to get some heals off use devotion aura and heal up and he wont be able to interrupt you. But since he has the talent to get rid of movement impairing effects use to to get rid of his silence instead of trinketing it, then heal up or something.
Other than all of that keep inquisition up, cleanse is diseases as much as you can and try your best to stay alive until you've forced him to use his magic shell and then burst him down. With long arm of the law and seal of justice along with emancipate and freedom you should be able to kite him. It will take lots of practice to beat him if you've had trouble in the past so if you keep losing don't give up, keep trying new things.