You can either do this or carry around small elemental damage items Chocolatte sells to deal elemental damage without killing. If you come across a Mirage in battle that's much weaker than you and you want to Imprism it but have to deal elemental damage, unstacking can usually keep you from killing it in one hit. What seems to be the primary use of unstacking is quite useful. The game makes sure to tell you that you can unstack in battle but the uses of doing so aren't that obvious. I really can't emphasize enough how important abusing Defend is.
There are some minor gameplay-oriented spoilers in these tips but nothing major - no storyline spoilers, but a slight spoiling of the "newness" of playing. Not only because you get new Mirages to add to your ranks, but also because of the game's super-reliance on elemental strengths and weaknesses. Unlike most other Final Fantasy games, WoFF demands you change your line ups around frequently. This tips guide is for those new to the game who are having trouble surviving and adjusting to their enemies.