Equip Colette with the EX Skills Defend (EX-Gem lvl 1) and Immunity (EX-Gem lvl 4). This will give Colette the Compound EX-Skill Reflect. Get your Luck stat as high as possible by equipping the Tambourine, two Rabbit's Feet, Lunar Guard, and Bridal Gloves. Get into a battle with one of the following enemies: Undertaker, Coffinmaster, Efreet, or Carnivorous Plant. Each of these enemies has a move that will pick you up in some way. When they do, if Reflect is activated, Colette will become stuck. In the case of all of them except Efreet, Collete will be stuck inside. If she dies while inside, she will be invisible upon revival. Other characters can get stuck with Colette if they are grabbed and Reflect is activated. Unfortunately, when any characters that were stuck are revived and can move around again, they will be "teleported" back and will be stuck again. Also, sometimes when the Carnivorous Plant picks them up, they can get stuck right outside of its mouth and not become invisible. Videos here and here.