The gimmick of this fight is that the Devourer of Nations will use a move that will add like 7 turns and will start to demolish you with attacks.
The best way here is to build your team around dodge or reflect chance, so you can quickly drain all his moves.
If you are smarter than me, Heismey is probably the best character here. But I didn't have him leveled up enough, so I used equipment that blocks fire for most characters and also stacked repel magic and physical damage chance on Hulkenberg.
It was a bit more RNG based, but it went pretty well. Though, it took a lot of time.