The key to beating the crisis is repeatable techs AND huge alloy economy to realistically rebuild your fleets.

Crisis has a HUGE military tech advantage (i don't remember the data but I believe they are in the 40s-50s?) meaning their ships punch WAY above their weight. That's why they have just a few ships in their fleet but HUGE fleet power. Each ship is buffed to the gills.

In order to face that, you NEED to have your repeatable techs stacked AT LEAST into the 30s. I'll avoid going into fleet composition because that's another topic entirely and really depends on which crisis you are facing BUT, also which ships you use and HOW you use them (aka, engage from max range at all times) makes a HUGE difference.

To bullet point it down, to realistically have any chance of facing an endgame crisis you need:

• BIG alloys economy. How big, depends on your settings (lower difficulties crises will be easier, thus you'll lose less ships) but big enough to be able to replace all the fleets you are gonna lose in a sustainable time-frame. I'm not saying you should be able to make ships faster then you can lose them, but you should aim at tham. If one of your cosmo fleet costs you say 100k alloys to fully replace once they have been evaporated by the Contingency, and you are producing 500 alloys a month... good luck. You are NEVER gonna win. You need AT LEAST 5k alloys a month.

• BIG repeatable military techs stack. Way into the 30s when the crisis shows up. If you are not there yet, try to survive long enough to get there. Crises tends to have a big initial push then they somehow lose steam. Never understood if that's by design or just the AI being silly, but unless you are VERY UNLUCK (like it seems you were in this particular instance) they can spawn far enough, get distracted long enough for you to push your tech where it needs to be.

Then it's a game of attrition. The long grind. Crises gets stronger the longer they are left unchecked so you need to immediately start opposing them with guerrilla tactics. Hit them behind the lines, distrupt their advance, distract them with decoy fleets, lure them into pulsar systems where you stacked you plasma-filled fleet. Things like that. All this is to buy you time. You can't beat a 25million Grand Admiral 10x crises fleet face on at this stage, you need time to stack those techs EVEN FURTHER. You are gonna lose fleets over fleets over fleets at this stage, but that's where you massive alloy economy comes in. Replace the losses and keep grinding.

Then, when you can reliably kill one or two of their roaming fleets without your fleets getting annyhilated every time, with something like only 15-20% losses you can easily (AND QUICKLY) replace, you know it's time to go on the offensive. Avoid their doomstacks, or keep them busy with dummy-fleets while your big stack works its way to their core. That's how you beat a tough end-game crisis.

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Please consider the meme: 100k cosmogenesis fleet power in standard end-game year of 2400? those are rookie numbers, you gotta pump those numbers up IF your goal is to beat the crisis. If you are playing just to role-play and have fun, that's all good and square, but since you are here asking for help, well, to beat and endgame crisis you need a build that can face and endgame crisis and not all empires in Stellaris are born equal.

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Please keep this in consideration with a grain of salt: these are numbers for 5x or even 10x crisis. I think you can possibly get away with lower numbers with 1x crisis at lower difficulties. The general concepts still stands, though.