in a party, grinding from 16 or 17 to 20ish on cactuars west of Horizon camp is quite viable. You'll get your fair share of chains. If you're duo or solo, better wait til 18-19. 100 needles can still hurt if you're alone.

After that, move southwest of the big crag with a pathway across it for cactuars between 23 and 26 I think.

Once that is done, there are even more ranging from 25 or 26 to 29 a bit west or a bit south. Can't quite remember.

you can eventually move on to Mitelings just a little north of Nophica's wells when you reach 27-29. These yield massive exp but can link nastily at times. They respawn fast even if you kill them all off in a burn, which is excellent because you get chain 5-8 before they are all cleared. You wait a few seconds for them all to respawn and then you just kill them all again for another chain.