This one is pretty tough
Byleth can rewind time 13 times (via divine pulse), has a decent AP advantage, effective attacks (bane of monsters), has pretty good healing (via crest, items, healing focus, magic, etc), prevent counter attacks, arguably a speed advantage when injured (via vantage), and a much better range advantage.
Garou on the other hand has very good enhanced senses/instinctive reaction, very high accelerated development, better in close combat, Low-mid regen, technique mimicry, type 2 immortality, and if prolonged long enough, should be able to turn into his awakened form, MUCH better stamina, and has great pain tolerance.
Garou would probably immediately aim for vitals such as the heart, but the thing is, Byleth's heart doesn't even beat so that would make it a lot less effective. If Byleth uses anything that doesn't involve weapons (such as magic and certain brawling combat arts) Garou can mimic them. Even if Byleth were to get a good hit on him, it wouldn't do too much since Garou has great pain tolerance and much better stamina. (not to mention low-mid regen). This is assuming that Byleth can even land a hit on Garou, as he has instinctive reactions. In short, Garou would probably stomp Byleth first time around. That is, first time around.
Byleth has 13 divine pulse uses, so that should be more than enough to get around their enhanced/instinctive reactions. Seeing how the sword of the creator has the range to carve mountains in half, they should also outrange. It's not like Byleth can't tank any hits either, as they have lots of healing, and can possibly even heal all the way. Byleth can also prevent them from counterattacking via windsweep, and has bane of monsters which is effective against monsters. Byleth shouldn't have too much of a problem with the regen, as they can easily decapitate people or even tear them into small pieces. (as seen when he basically split edelgard in half and the slicing animation in sublime heaven.) Pain tolerance can be helpful, but it doesn't really do anything in terms of reducing damage. Also, if it counts, vantage makse byleth always attack first when injured. Byleth will probably be aware of his awakening in the first few divine pulses, so they will know to take them out quickly via decapitation or shredding them to pieces.
I'll vote Byleth high diff.
if only byleth had chalice of beginnings