Taimumari: Complete Edition

I found a Youtube video that shows a very good spot to grind:

All credits go to RetroSteve who recorded this video. He found and shared the method.

It's on the first level and you use the bomb to destroy the red blocks on the upper right corner near the start of the second section. I activated the 99 lives from the cheat menu but I had enough from beating the game.

The stars ARE cumulative. You can spend as many as you want. Make sure to get the magnet update to make it easier to collect the stars during the levels. I grinded there after beating the game. Load your auto save that is created after beating the game.

Activating the 999 stars cheat, spending some on the shop and activating 999 stars again does not unlock the achievement. But it helps to get all the upgrades - which makes the game faster to complete.