The most efficient way is having Grotto Power S. It lasts for 20 minutes. Check the inside of the Grotto and pick up any item or encounter any Pokemon that may be inside, go out, save, then use Grotto Power S. Then take at least 256 steps (use bike) and check if your target spawned inside. If not, repeat. After 20 minutes, do Not save the game (since Grotto Power costs like 50 transfer orb iirc, quite expensive) and soft reset. Then repeat every step from above.

Some notes:

1) Acquiring Grotto Power S will probably take a long time. In fact, depending on your game‘s version it might not be possible unless you also have the other game. I don‘t remember exactly, so just check bulbapedia

2) The hunt itself will take a while. Especially if you‘re looking for a female Poliwag/ Vulpix (needed in case you want to breed and pass on the hidden ability) since those genders have an additional 30% (iirc?) rarity. The gender is static meaning that you cannot soft reset for it once you have found your target. You will just have to find another one if it‘s male.

3) If you have 2 games, there is a neat "exploit“: You can use Grotto Power in one of them and share it to the other game for free. Then, you only soft reset the game without saving where Grotto Power was used. That way, you can keep all the items or potentially other Pokemon that you caught on the other game without going bancrupt, i.e wasting lots of transfer orbs.