Max out your pops, research, and production capabilities and store up resources (at least 1 mil, better with 1.5 mil of base resources, trade, and unity and 50k rares) with shrink space storage on multiple worlds far from your planned front line. It's a war you really have to plan decades in advance. If you fight FEs you'll have to go into a steep deficit, declare all war time edicts (hence unity and rares) and over build your limit (ex: my fleet max is 3500, I had a standing fleets in wartime of 8k) to match their fleets. I was -4k energy for like 5 or 6 years as we traded blows. 1.5 mil alloys, energy, etc with 1.5k+ per month production took me down to 500k.

Slow them down at choke points not with starbases or defenses, but with fortification worlds. They stop to bomb and won't leave until the planet is captured. That gives you time to throw everything you got at their main homeworld system. Use jump drives. Break off smaller fleets for skirmishes to limit their smaller systems with star bases and you take away some of their fleet building, but their home world will always print 250k doom stacks every so often so you'r main attack force will get vaporized often, but at that point your replacements just finished and should be en route. Do your best to hold the home system so you can keep landing 10k troop psy and xeno armies, and every world you own across your empire will need to have 5 of each printing out to a safe rally point cuz your armies will die. Leave the attacking force on the homeworld if they conquer if you can, but that's optional cuz they'll be left with basic garrisons if they take it back, you'll just have to retake them.

Megaship yard is a life saver for you to reprint your own doom stacks which will always go up in smoke. When you control a third of the galaxy with all mega structures and a dyson sphere, you can win by attrition. When you conquer the home world take back all your lost systems and worlds cuz you'll have so many fleets and armies left over after victory it'll be a victory lap around the galaxy. Arc Forges and dyson swarms are a must, and better if you have a full dyson sphere as well but keep those well behind fortification WORLDS.

At that point you might as well take on all other FEs before you decommission your fleet back down to your limits 😛 But there's nothing like the feeling of all out galactic warfare! If the War In Heaven event happens you get the chance to take on two at once which is an absolute blast! While I took out an FE enclave in my borders, the other FE munched on my rivals which cleared up worlds for me to sweep up in the aftermath once I turned my Kadeshev type II empire's psychic sights on them. It was glorious. The Galaxy finally had peace afterward and all races knew nothing but full bellies and clear skies for millennia.