Don't teach it Mimic. It evolves the next level after learning Mimic, which is Level 15.
If you want Mimic for whatever reason:
- Let it hold an Everstone.
- If you do not have an Everstone, Press "B" every time it tries to evolve.
Everstone Location for B2/W2:
Driftveil City - From a Butler on 23F of the building just east of the water.