Go up the stairs or down the stairs when he does that - the blades will not follow. And if you can’t do that, there were some boxes on the upper platform at least - jump on them. Every character should be able to do it. And the Lava on the back part cannot hurt you!
Though all of this is moot if the… Slow(?) he throws out at the start of that skill hits you and the very first blade hit throws you out into the void.
Overall the whole Rendain fight feels like a cheap way to make something challenging.
Not to mention how boring it actually is to not get any new bosses, but instead being forced to fight all the past ones. Sure it’s a cool Idea, but since the whole game has such a serious lack of enemy variety, it just does more harm than good.

(Though this can be ignored if you just want to get the mission done)