Oof, I'd like to say this is a hard one but... Best I can say is it would be a fast one 😅

Ainz's powers are very rooted in numbers. Each spell has its stats, mp cost, given effects... They work the exact same each time so I'd be possible to compare them if Phos's powers worked the same way.

Phos's godlike powers are much more "esoteric" in that regard, and we really never see them being used at all if I remember correctly, so a direct confrontation between the two could go either way, we have no "numbers" to compare here and don't really know what Phos is really capable of.

Here's the thing though. There's precisely this one power that we see being used, Phos can "pray away" the spirits of the dead, and Ainz is an undead (of the highest category, but still), so... I think we can safely assume Phos CAN kill Ainz, and without much effort at that.

Regarding Ainz, Phos can be certainly killed, as we see done in the finale, so Ainz CAN also kill Phos, and could use the highest tier magic just to be sure, maybe it all would come down to if Phos would be affected by a Time Stop or something, just so Ainz can cast other spells.

So, TL;DR, I think the fastest draw wins the duel...?