This is the best way I have found to get the "King of the Rings" Achievement, get 200 rings in a level.

First, make sure you have the stage select extra unlocked: if not, you need to get <5 minutes total on time attack for the special stages.

Then go to extras > stage select. Select 1-1-C, this is Palmtree Panic good future.

Since it's the good future, there will be no enemies, so you can take your time getting as many rings as you can. You should be able to get around 160-180. Then go find a past sign, and go to the past. You will easily be able to get the remaining rings you need here (although be careful of enemies now that you will be back in the present.)