Beating it depends on the unblockable set-up, as far as counter assaults go.
I've seen one where the unblockable set-up was done off of moves that actually connected as hits, so basically there would have been no way to counter assault out, since the player was not blocking the hits before.
In the case of an unblockable set-up where the player is already blocking, since most counter assaults have a good number of active frames, if the player recognizes the set-up, they can counter assault right before it and catch Relius as he going in. This can also be baited by Relius too, all he has to do is barrier block instead of going for it while Ignis continues to attack. You guys should get a list going of these set-ups, I'd like to see more of them.
Man I am going to hate fighting you guys.