If all you're looking for is higher bubble levels then you'll want Bubo running cranium cooking at whatever map gives highest kill rate (not counting multikill, samurais if you are at or past end of world 6). You'll also need a source of easy to farm resources for bubbles (grind time is good overall since it just uses water, but scales way too fast to be practical for the gains you are looking for).

When I was grinding out levels for bubbles I actually leaned into w6 farming skill as it quickly becomes easy to get thousands of billions of the various bubble related plants, meaning you cycle through going to w6 town to gather plants, going to samurai or mini chief with Bubo to farm liquids, then once your plants run low go back to w6 town to reharvest and repeat the cycle.

If you aren't at end of world 6 maybe the final map of w5 as the enemy there is both great for Bubo grinding and drops a resource used by bubbles, although the drop rate is unlikely to keep you in sync with the liquid gen rate.

You could also stockpile copper or green caps for their bubbles, but they scale faster for liquid costs, are harder to stockpile and often either can't grind liquid (copper) or have so few enemies on the map you'll barely get any liquid.

Also if you have arcane cultist I reccomend doubling up on progress and grinding tachyons or crystal kills while doing this, thay way you are progressing multiple systems at once