I used the catalyst Obsidian Warcastle to beat Eternity. This one is a lot tougher than the other two bosses but if you take your time and keep your distance its fairly easy.
Just make sure you boost into the bombs it drops that have the countdown timer otherwise has a ward which deals massive damage to your ship.
Then just use your LT + X cannon whenever its charged to deal decent damage. Don't get close to Eternity untill you have LT+A or LT=B both charged and ready to fire.
I show it at 42:30 in my full run-through of the 1000g completion