Well, I've just got this achievement after trying different things - seems the answer is, you don't have to get them in order BUT, you need to get the potion THEN FINISH THE LEVEL, so that it says 'Potions found...1' at the end.
It's no good dipping in to an already finished level, finding the potion then quitting the level.

Trouble is, if you've finished all the potion levels, and still haven't got the achievement, (which means there's at least one level that you haven't got the potion and playd to the end of the level) there's nothing to tell you which level(s) you need to go back and try again. I had to just go back to the start and work through them all until the achievement unlocked (for it was level 4 I'd missed).