Alright, so first thing I'm seeing is that we need more information. With the first few paragraphs, we know Declan has been somehow involved in murders, works for someone, and he's going to do something somewhere with a lot of money. Jason sees his face, but your reader never does. There's not really a hook. Sure, murders are bad, but your readers are only going to care about it enough as it's shown to matter. Han Solo shoots a stormtrooper, go Han! Han Solo shoots a stormtrooper and it shows him keeling over as he hacks blood into his visor, shaking hands reaching for his rifle as gore splutters out of the hole in his chest before finally collapsing with an anguished, doomed cry... less go Han. Same kinda concept. Declan's killed people. Okay? Bad people? Vamprina's an assassin, but still more good than evil. Why should we be rallying behind Jason? Show us the face of evil and the repercussions for the actions.

Second, and still kind of following the details bit, but be sure to describe your setting. If it's just a place, guards to me are more akin to mall cops than police officers, so while it makes sense they'd be trying to deal with whatever happened, having them open fire it going to cause some backpedaling.

Next, we've got timing. In Shadow of Mordor, I can assassinate a single-file line of uruks no problem, but in real life, people pay attention to potentially dangerous sounds.

Then, Declan appeared out of nowhere behind one of the guards and hit him with a metal bat. He immediately hit the guard right to him and then the one left of him. After that. he quickly pounced on the last guard before he had the time to react. Before the other three had the time to get up, Declan got over them and knocked them all out.

If you hit me with a stick, I'm making a sound, but even if we're talking speed, you still have to swing the thing. Let's say this whole thing took two seconds for all of the first three, but the last guy still hasn't even glanced over his shoulder? If you're trying to make someone look good by dispatching multiple enemies, you have to walk a thin line between character being sweet and the guards just being terrible. If it's the later, it's distracting.

Next, we've got some logic issues. Anytime you make your reader pause and thing 'Well that was weird' it fractures the flow. Try not to force things, and make sure you're following a natural train of thought.

First off, more minor:

He knew of course there was a “logical” explanation to this, but still impressive.

Jason is literally magic. Can't magic be an explanation?

Next is more important:

He got near enough and shot a fireball at Declan, who was searching the guards’ bodies.

I mean, it's not Bethesda, is this something Declan really has time for? He just did something high profile enough to warrant open fire in a public setting, and then he knocked those people out with a baseball bat. His ride is gone an the police are coming, he's got to go. I know that Jason and Declan have to fight but, there are better ways to stall.

Declan was more tired than him because he just committed a crime or something like that.

To be honest, this just comes across as super lazy. Jason has been staking out this are for a week. He knew Declan was going to hit something, so he had to know something, and he saw him come out of somewhere under fire. At the very least, Jason has very good educated guess what just happened. If I hear gunshots and see someone run out of a bank, it was robbed.

He forced him to turn to an alley he knew led to a dead end.

More of a missing detail thing, but how's Jason forcing him down this ally? If you're sprinting after me, you're stuck with my route, so how's Jason lobbing those fireballs to convince Declan his path? It could be a good chase scene you could've shown us. As a side note, did you confirm with Wolf that Declan can phase through walls? Looking over his powers, I can see it's mentioned he takes a kind of spirit form when he makes his fake, but that's not something specifically mentioned. For the next while it's a whole lot of running without much payoff. We're hearing that Jason is trying to catch him, but we never feel his frustration build. Show his face? How's he taking all these near misses? And when Declan finally gets his man-catcher, is Jason worried at all? It feels like he just looks at it, and then remembers its name. When tides change, those are the moments more than any other that you need to take slow, show bit by bit and be sure to convey what the characters feel during them. The entire long chase, you have Delcan using his power to evade Jason, Jason constantly using his elements and then at the end, Declan is just spontaneously forgets all of that, and kinda lets himself get caught.

After that, we go back to more missing details:

But Jason had learned how Declan’s powers worked. After he gave him in. they informed him about his abilities and other stuff.

This is another thing that there's really no point to withhold from your reader. If Jason knows it, there's no reason why your reader shouldn't.

All in all, keep your reader informed, show how your characters feel, and be sure that there's a reason why they're acting the way they do.