Well, somehow I managed to get B's on all of the Business District stages except for stage 2, in which I made a C. But I'll tell you my strat for stage 3 (NTSE Plaza). On the very top of the map right by where the gate/end is, on the left side, I built Wee ninja huts, with a Tea hut in the middle. On the right side, all sniper huts, with the Guard (range) tower in the middle. Upgrade Wee and sniper towers when you can. And on the strip below those, you can build frost and some Business ninja to help slow down more (you don't have to upgrade them). You might want to try replacing the very last 2 towers at the end (right next to the gate) with Anti ninja because those blobs still get to do some damage to my gate and therefore ruining my A (I think).
I'm stuck at 7/4. Those balloon devils are mean.