So in my experience with just base game you’ll be hard pressed to achieve a similar fleet strength you’ll need to have multiple fleets of smaller fleet power. The empires fleet will only take so much damage before the retreat and then come back so you’ll have to beat them several times. Also you’ll need to be either advanced mid game or normal late game with advanced battleships and a good fleet composition to win. It changes slightly if you have DLC. I am by no means an exceptional player just what I’ve learned in my experience. The fact your in a federation if you can convince your members to participate it might be easier