white already has no problems with it, with or without cast out. lots of removal.
@Laeuftbeidir is correct; the bug here lies with Prevent Damage, which is the evergreen icon on all god creatures.
You can verify this with the spirit that gives prevent damage on activate, or by using Avacyn.
Not creature removal, and not in standard once Kaladesh rotates. Tragic Arrogance won’t work if Rhonas is given reach since he’ll be in the first creature slot. Suppression Bonds and Hixus could work but are higher rarity. Cast Out works, but if it’s against Kiora, all she has to do is cast Vizier of Tumbling Sands, and Cast Out stops working.
But, more to the point, if multiple colors have no way of dealing with the issue, then it’s a problem with the combination of cards, and the devs will need to find a way to ensure that other combinations, like Oketra and Knightly Valor don’t cause a problem.
I know the “super prevent damage” will be fixed soon, but I’m having a ton of fun with Oketra in my Gideon1 deck. Get a couple of cheap creatures out, use first ability on Oketra, never lose game. Had an Ob deck run 6 kill spells in a row into Oketra, and she laughed almost as much as I did 