Are you playing the English version?
Stage 2 is actually considered one of the most grind-friendly.
First, don't rush enemies let them come to you.
Second, guest units (Guncannon, Guntank, Core Fighter) have higher stats than enemy units, so you can use them to trim down enemy numbers.
Then you hit what's left of enemy units with the guests and finish them off with your originals. At this point in the game you should already have access to the Zani, it's basically a Jim with an insane long range rifle: buy a couple.
Third, use squad attacks and master squad attacks (the manual should make things clear for you) and double tap every enemy (shoot everything using two units, not just one). Finish off the flying magellas with melee, otherwise you won't have enough accuracy to hit them.
Don't waste your time fighting the Dopps in the sky, they'll come to you.
Always check on enemy units' range, since most of them have blind spots.
