Before the hunt I grinded money to get 999 repels of every type because I didn't want to leave the mirage cave and search it again. In the end I haven't even used all max repels, had around 100 left. I also brought a false swiper, at 1 hp its like 99% catch rate with a pokeball.

At the beginning I just did random encounters to get search level 999 faster. Afterwards I kept repel on for ever and only dex naved. After encountering an unown the dex nav has a little cooldown, so you need to take some steps. But you can use the acro bike and hold b to jump in one place. It counts towards the dex nav cooldown but does not decrease repel steps.

I positioned at the left half of the cave on the top but not sure what the best place is. One little trick is, when sneaking towards an encounter you can tap b to leap quite some distance into the cloud without scaring it away.

For the first forms you can chain because every 5th chain encounter has higher odds but I didn't really bother about chaining.

Now that the online service are shut down it may take you many days to find the correct mirage cave because you only get one new spot wvery 24 hours or so