For those of you who want to level up your units quick, farm level 1-3. After you defeat Char's Zaku and the Gau carrier shows up, just destroy the Zakus only until time runs out. You'll fail the level, but keep the exp. Rinse and repeat until you get to whatever level you want.
Make sure to set your partner to "Evade" so that they don't shoot down the Gau!