EDIT: Reread Shadow and Fortune to give me a better idea of the matchup.
Lucian:
Gear: Dual lightslingers built specifically to take out undead foes.
Training: Culled undead for many years with Senna's help, then learned dual-wielding after her death.
Powers: Mostly weapon reliant, but has the ability to move very quickly as shown by relentless pursuit. He also is strong enough to wield the lightslingers which were originally used as a two-handed rifle. This is similar to Ike's (of Fire Emblem and Super Smash Bros) feat of wielding a two-handed sword in just one hand. Correction the lore states that they were originally pistols, why did they carry one each then?
Weakness: He is a regular man. He may have a bunch of training and strength he is fighting a twisted spirit. He will be vulnerable and have to outclass thresh for a while to even have a shot at winning.
Thresh:
Gear: A Spectral Lantern which contains a ton of souls captured by Thresh. A large hook with which he can grab and torture his victims.
Training: Before death he was trained in the use of magical artifacts and experimented on them. Exploits include tearing out pages of a living book, and scratching a mirror which was a mage's look out into this world. He has great experience using chains and other torture devices.
Powers: Ability to store souls. He is also undead giving him remarkable durability and resistance to normal weapons as seen in the harrowing of Bilgewater. As an undead he would also have incredible stamina. He is also extremely smart figuring out new ways for torture as well as being cunning enough to outsmart Lucian and Senna and capture the latter. Can generate an ethereal prison as shown with "The Box." Also according to the video above thresh can fire blasts of energy. He can use his lantern to protect him from harm, possibly sacrificing his souls however to do so.
Weaknesses:
Thresh was never meant for combat he was an experimenter and tormentor. Thresh barely ever confronts his foes directly, choosing rather to trick them before tormenting them. He also might have a bit of a pride issue, he sees his experiments as a higher form of thinking and may fall because of that.
Conclusion:
This is technically the second time this kind of fight has happened the first being detailed in Shadow of Fortune
In this fight Thresh has the upper hand initially being able to block shot with his lantern and once he gets a hook on Lucian it's all but over. However thanks to Senna Lucian gets past the lantern shield and Thresh is forced to flee the scene lest he be destroyed by the lightslingers.
A second round would be a match of Lucian being able to dodge until he could get past Thresh's defenses. His weapons have been shown to seriously hurt Thresh to the point of making the specter flee. Lucian also has the advantage of Senna being able to somewhat communicate with Lucian through the lantern. If Lucian can get past the lantern without being hook he could absolutely win as Thresh relies on his magics and fear to win and has little combat prowess of his own.
I would give the match to #TeamLucian with a 55:45 matchup.
Come at me bro!
"Never hunger, never tire, never fear. Join her, Lucian!"
Team Thresh 
