So-
AP: Valev, by around a x4-x5 margin. So, if he gets hits in, he's killing really damn fast.
Speed: Samael, duh. Well, he slows himself down, but it's still impossible for Valev to have a speed advantage of Samael can just not be slower, kek. I doubt he'll abuse this, though.
Abilities: In a practical sense, Samael. Deathhax is a *****, and so is 1-A reflection shenanigans. But in an actual, fair fight, Valev has various abilities with cloning, time manipulation, and some of Samael's own abilities to make it incredibly difficult to really scuff him, especially while he has a x4-5 AP advantage. Samael's intelligence-based Precog will help, though, even if Valev is unpredictable.
Intelligence/Personality: Samael is incredibly intelligent, more so than any individual Valev. But his overconfident screwery, wanting to play with the opponent, is absolutely damning here. Valev will constantly be at the receiving end of an information line from The Collective, who will be more than willing to help put a demonic jackass in his place. In terms of skill, they're probably equal enough, though in strategy, Samael is self-crippled.
How I see the battle going: Samael attempts to Info Analysis and Power Null, both of which fail. Valev power copies, and notes that his opponent has a crap ton of methods to obliterate him (based on seeing things too strong for Valev to copy), and questions why. Samael, likely going to his cocky origins and more confident in his ability to win, makes a snide comment.
The battle begins, and Samael's supicions are very quickly revealed true. While incredibly powerful and skilled as a brawler, Valev is a brawler, and fights a normal fight, with stabs and punches. This definitely fits into the type of thing Samael enjoys, from what I've heard, so he definitely won't resort to the big guns anytime soon. Samael will find that actually landing a hit on Valev is incredibly damn difficult, especially one to do meaningful damage.
Samael's precog will doubtlessly help, but Valev's self-invented tactics and unknown magics/abilities will allow him to bypass it at points, being too unpredictable to properly predict with analysis alone.
Now, it comes down to: Can Valev bypass Revival, Deathhax, or the Crown? And the answer is... Yea, no. He can't. Or at least... not in a typical sense.
Valev will, be it with Yvonne's abilities or Eowyn's, learn of his Revival- or more importantly, how it has to be activated before death. Now, throughout the entire battle, Valev will not once use his time manipulation. Simply because, it's his trump card.
The moment he finds the perfect situation, before that revival spell goes up, he uses his copied abilities to weaken Samael's Time Manipulation Resistance and fires a Time Stop. Samael's resistance is very high, but given how many of Valev's abilities are limited in power only by how much mana is put into them, and that he'll have a weakened Samael null powered by his own abilities, this is likely to work.
Not for long, of course. I doubt a time stop would last more than 3 seconds before Samael's resistance frees him, or the mana cost to seal it becomes too much. But those seconds is all that matters. No, the tiniest blip, tiniest millisecond, enough for Valev to take his abilities and slay Samael, ripping the soul from his body, before he could ever activate a Revival. And with Samael's resistance sealing + very potent time manipulation in conjunction with amps and Yvonne's space-time alternate timeline screwery, I think he might just be able to do it. Not for long, but any amount of time, is time enough.
In conclusion, Valev pulls an ORW move and takes advantage of the tiny weaknesses of an unimaginably overwhelming foe. Samael's overconfidence, his tendency to mess with the opponent, and his revival needing to be activated, all working towards a final, finishing blow with a secret trump card.
Did I miss anything?