@zoe01,the thing is,this very Eagle Libre you faced, you might not face it again. The system always brings out new one, different from the one you fought with before.

Now to your battle, first of all the boss has these four abilities, Void armor, repair, thorns and inspire, pure defense. In a situation like this, you try as much as possible to reduce the number of monsters with melee attacks in your deck collection because of thorns on the boss. So you focus on monsters with ranged and magic attack at least.

Because the way the boss is set up,you must first of all depletes its armor before you can get to its health, because it has void armor ability as well. If you don't mind share the battle link replay.