320 peanuts can be beaten pretty easily by the first level, Battle of Shell City, if you know how to get the most out of the combo mechanic. You're looking at 485 peanuts in each optimal run for that level.

The other thing about this guide is that even when you quit before that notorious bit with the bouncy balls rising through the grates, I wouldn't put my trust in people to not accidentally die while grinding on Welcome to Zappo Land with the electricity and double flame tanks.

i think its better if you pick up the 3 giant peanuts of that level and retire right after the last checkpoint..or if you get at critical hp

Depends on how good you are at platformers you might find some different level. I recommend playing 1st level. Fast, easy and stable income of PBs.

If you're going for this achievement, I recommend doing it as soon as you unlock this level, so you can actually make use of these Jars, and since there will be some parts (like the part of the next stage and the final boss) that will probably make you lose some Jars. Note that you'll have to recollect any Jars you lose.