If your looking to gain Exp for most or even all of the your wonderful ones then the best place to do it is Operation 002-B.



The method is simple The water Slide in this level actually gives any wonderful one who rides it a decent amount of XP. if you use the wonder-liner to send all of your memebers into the slide they will all gain XP at once.



The Ferris Wheel right after does the exact same as the Water Slide, sending in your memebers with the wonder-liner will also in sure you collect all of the Space Vegetables on the ride.



after doing so you simple need to complete the fight in the very next section, once the fight is completed the game will save, and you can pause the game and return to the operation select menu and restart the level again to gain XP and Vegetables as many times as you want.

It's likely by the time you unlock the secret character from collecting Bottle caps most if not all of your Wonderful ones will be Level 4, so the pervious method won't be optimal for a single member.

To quickly gain levels with a single member I found the Megang enemies are a great source of XP. when standing close to a Megang they will attempt to step on you. using Unite Guts will send them recoiling back, each time you successfully refect them the current leader will gain a huge amount of XP, they will be brought from level 1 to level 4 in less then 2 mins if you simple do nothing but use unite guts on the Megang stomp over and over again.

It will work with any Megang in the game, but 2 very close together can be found in Operation 001-A when on Hard or 101% Hard difficulty.