How to beat Aeldari Sanctuary special mission with Tyranids.

How do you do it? There's so many cruisers and that huge Eldar ship that just destroys everything

So I brought about twice the command limit for this one. First contacts you get are the Imperium ships in the bottom right side. They have a lot, 3-4 BC, about the same each for Cruisers and LC. The entire time you fight them, you are getting bombarded by the craftworld artillery, forcing you to save your ship's rushes to evade, although it does sometimes catch an enemy ship. I managed to defeat the imperials, after losing a bunch of ships. Then it's a bug hunt to try and find the 2 eldar fleets, under fire the whole time. I did find one, up in the top left corner, but by that time I was down to four ships, and the 4 or so Corsair Cruisers plus the omnipresent artillery finished me off fairly quickly. Especially since there seems to be a cap on reinforcements, where you don't get every ship you lose replaced, just the first couple - three in my case.

I imagine the other Eldar fleet is in the bottom left corner.

But this mission is seriously difficult (as is the Ork one with all the space whales). You just get worn down with wave after wave of fleets that are each equivalent in size to your own.

And that was on easy difficulty.

Any tips would be appreciated.

Pretty sure there is no cap on reinforcement fleets, im currently on an imperium hard mode campaign and almost every invasion fleet is like 3800 strength, and yes i have to kill ALL of them, they come on as fast as they die and its a total cluster***k. Just bring the absolute max you can with 3 full fleets and overwhelm them, if its still early ish in your campaign and you havent reached the 1k limit per captain yet then maybe wait? i had to put off killing the ancient one for quite a while till i was strong enough level.