Realistically, i think Ike, Roy and Byleth are just the ones remaining.

Die instantly -
Corrin/Robin/FE8 Twins (since they got no super impressive feat with their divine weapons) Leif and Celica maybe the fates lords too

Die later - All Falchion Naga users since they are all comparable and equal to one another, Alm can rewind, which might make him stay alive the longest

Die ever later - Lyn since Sol Katti is not as strong as the other elibe weapons, and all 3H Lords and Shez die here too since they are just rewindless Byleth abd usually depicted as weaker than them..

Die after that - FE7 lords, Lilina, maybe Micaiah due to less 1v1 experience

Top 3 - Ike, Byleth, Roy
Roy and Ike can heal, Byleth can rewind, but the attack potency difference is too high for them to really do anything.
Ike skillschecks em both.
But Aureola scaling based on the animation is 1000x FTL and Roy just... blitzes and is too durable and strong.

So yeah, Roy wins the battle royale imo

Heroes/Crossovers vaporize everyone else by just being there just fyi

Fun things that could happen - Marth/Chrom/Lucina have Shield which COULD give em Darksphere evil mind control in a very out of character dick move, which only few lords resist/can live without their souls (Elibes due to fighting fire dragons that could destroy Idunn's soul and Celica) and Eirika/Ephraim can use the stone of rausten which they had on em at some point and seal any one persons soul before going down