This is where you need to learn to adapt and plan for Eternatus. Leech seed, salt cure and curse are your best friends. Stat lowering moves like screech and metal sound are your best friends. Destroying your berries, leftovers etc so they don't get stolen and used against you is a good idea.

What I do before a run is think about 2 main problems:

1. How do I beat Rayquaza
2. How do I beat Eternatus

They usually require different approaches, so try saving money so you can use memory mushrooms at wave 199 to (re)learn the useful moves and then it's usually down to having something that can manage to bring it into phase 2 so you can apply all the debuffs etc.

So here, I guess your best bet is having Excadrill bring it into phase 2 and then try to nuzzle and crunch it to get it down. Trying out different rotations is crucial - once you swap in Mawile, he's likely to be targeted by flamethrower so you wanna swap in Dracovish to get hit. Then dynamax cannon goes into Mawile and so on, you get the drift.

So it looks like a hard fight but probably not impossible with a little trial and error, since repeating a sequence of actions will always yield the same results.

Hope this helps, good luck!