cant beat devourerer of nations, always using shriek no matter what is equipped

been trying for about an hour now and after learning he will always get 8 turns when someone has repel/null fire I took it off my party but he it still using it every turn. his turn order has always been dragon's wrath buff> all reset buff remover> shriek for 8 turn icons>random attacks. I was told he wont always do this but it has been nothing but this as long as Ive been fighting him. does resist fire count for it to auto trigger or does proclamation and him not having max attack buff make it so he wipes and buffs himself then will always use it or what?