Now that the final showdown is posted (but not finished) and both heroes have reached their peak of power, Finn and Leah can be comparable. These versions of them, at least. Apart from being more underrated than Ron Weasley (no small feat), both can make a great team because their skills complement each other. So, how does my version of Leah surpass Finn and vice versa?
Leah: Physically stronger
The most basic asset in a fight is strength. How hard can someone hit the enemy. While Finn is pretty strong and can hold his own against a vampire unarmed, Leah, even without her wolf form, can punch out many vampires and demons, rip the jaws of a demon off and even hurt Alastor with her punches, who is in a much higher level than Leah or Finn. Finally, she beat a Sue, Takara, with one punch, while Finn had to use his head.
Finn: Arsenal
The main reason Finn can catch up to Leah's physical strength is not his own, not even close. It's his weapons. Leah's weapons are a wooden stake, Garlic Bombs and a steel pipe (recently upgraded to a titanium crowbar)... two of which are only very effective at a single type of monster. Finn, on the other hand, has detonators, a real lightsaber, a blaster he knows how to use very well and a Microwave Beam. The blaster alone is superior to Leah's arsenal.
Leah: Agility
The Force allows Finn to enhance his speed and dodge attacks a normal person can't, and can even sense the movements of vampires and werewolves and counter their attacks. But Leah is actually faster than a normal vampire or werewolf in human form, which combined with her aforementioned strength allows her to perform rapid-fire fisticuffs as her main attack. She also gains the ability to give a punch so fast it is stronger than normal and emits a sonic wave. In wolf form she is even faster and can even attack up to seven enemies without even being seen at full speed!
Finn: Range
Leah's Sonic Overdrive and Garlic Bombs are ranged, but not very powerful, and she relies mostly on her very strong melee attacks. Her speed can, however, allow her to catch up to ranged fighters and punch them out, but she still mostly has to reach her enemy to strike, it's just easy to do so. Finn, on the other hand, can benefit from long ranges, since the weapon he is and always was best at is the blaster. He is skilled with his lightsaber, but not to the level of other Jedi, however few can compete with his blaster skills. And even without the blaster, the Microwave Beam and especially the Force allows Finn to fight very well from a distance, even guide his detonators in a straight line and push his enemies backwards to not let them get close.
Leah: Regeneration
Both warriors are very durable, Finn moreso in comparison to his own species, but nearly equal. However, being a werewolf, Leah has something that can let her last longer. Being a werewolf, she can regenerate health over time far faster than a human, even during combat. Coupled with her velocity, it makes her very hard to kill, as a lucky hit hardly ever sticks. This has saved her life more than once against enemies. Finn simply has no such thing.
Finn: No obvious weaknesses
Finn is not without his weaknesses. Unarmed he can't fight the enemies who are easy to him normally, and his compassion has backfired more than once. Leah might be far stronger without her weapons, and some weaknesses like overspecialization to vampires are even overcome later, but she has a fatal combination of two: Aversion to silver and not a lot of ranged attacks. Which means enemies made of silver, which can burn her on contact, limit her skills to less than a quarter of them for her to get through.
Leah: Wisdom
Finn comes from a Galaxy far, far away. He knows a lot about the Empire and First Order, but these are not the enemies he faces in this story. In fact, the first enemy his prior knowledge helps him against is the Loyalist, who has a fighting style known to his Galaxy. Leah, on the other hand, knows everything about the primary enemies, the vampires, and against them she is the most effective fighter in Stormwolf. Also, being an Earthling, she knows a lot about Earth in general.
Finn: Intelligence
Yes, he is naive and yes, Leah is wiser about Earth and bloodsuckers. Leah also has moments of intelligence, such as as when she overcomes her weakness to silver, lures Takara to get the only hit Leah needed and the entire Armilla fight, and more, but Finn is even smarter. Not only does he usually beat her at chess, he can adapt even faster than her and knows when not to fight. He also is good with mind games, being able to sometimes guess what his enemies will say next, and even when he fails he can still improvise! And, finally, he is completely immune to mind control. Says a lot really.
Leah: Transformation
In New Timeline, Finn can transform to literally nothing. Leah, on the other hand, can transform into a wolf. This allows her to do many things, like getting herself and Finn (who can't pilot but can ride) through long distances, increase her strength and speed further, and even has special abilities like the fearsome Seven-Sided Strike.
Finn: The Force
Here it is. The true reason Finn can catch up to Leah and his high-level current enemies. The reason he has special abilities equal to Leah's. It all started in Exegol, when he used it to immediately pinpoint the weakness of the entire enemy fleet, managed to sense the elite Sith Troopers and fight them off with no injuries and end up taking out the entire fleet and saving the whole Galaxy. Since then, he trained. And trained. And trained. He does not have all Force abilities, not even half of them, but he can sense enemies behind him, the very fast and without smell vampires, guide his projectiles to home on enemies, increase his reflexes to the point he can keep up with monsters and use a saber well and lift objects too heavy for him otherwise.
In durability and special abilities they are so close there is no clear winner.