Get "Very Rich" (get 500 metacoins), has lowest % and the only one that's basically missable in a normal playthrough imo

If you missed it like I did there's a coin detector far north in the Nexus (above the portal to the Anodyne 1 Land), speak twice to the slime and you'll get it if you have enough coins

The slime also tells you how many metacoins you got total

It tells you if there are metacoins left in the current 3d area

Not actually very helpful for the achievement since there seems to be around 600 metacoins available, so you can safely miss a lot. Still useful to know if an area is empty before even searching

Many of the coins require to go beyond where it's "intended" to go, by climbing mountains for example.

You can also clip through walls ( https://gtm.steamproxy.vip/app/877810/discussions/0/3185654583884614291/ ), I think this is required in for example Desert Area South to get past the barrier.

A trick that also may be useful is riding walls with the vehicle form: you don't lose altitude when running towards a wall and accelerating as the vehicle, which is useful on slanted surfaces to get higher than you could otherwise