Well, Valflame made Arvis capable of fighting another
6-B and his army, which includes other 6-Bs (Ced, for example, is canonically the son of
Lewyn (Fire Emblem) according to Thracia 776, which makes him capable of also having Forseti AKA a 6-B key) and Arvis has fought fliers before as well as people who have dealt with fliers
I'd definitely say Arvis wins if he was allowed to use Nihil (It nullifies abilities of the weapons and the user, but not their strength), but without it, he gets destroyed by Matter Manipulation and Precog
Actually, I think with Nihil all that would be nullified is abilities granted to him by the Helmet of Nabu, since verse equalization would mean it's sort of like the Thief Sword, which also grants an ability to the user, but is nullified by Nihil, but since it never nullifies tomes, I'd say his ice manipulation would still stay, since Blizzard is still capable of being used against Arvis, which actually freezes the opponent in FE4, as well as his weather manipulation, since well...tomes can be used against Arvis...
and the power granted by a weapon isn't nullified by a weapon/item isn't nullified since well....Holy weapons are still 6-B against Arvis canonically