As said on the tin, get all the minikits in one level. Grab the easy ones when you can in the level, with the characters you have, but for most, if not all, levels, you'll need to return when you've completed the storyline, and have the characters to get the rest of the minikits.
There's no easy level I can suggest for this as they're all the same, as in, you'll get a couple minikits, but won't be able to get them all first run through. However, I would recommend IGN walkthroughs as they show you WHERE the minikits are and give you a guide for it.