Easy money
There is an easy way to collect money in the level where you must collect farm animals. Instead of rushing to collect the animals, kill off the bird-like robots. One way to do this is to shoot one from afar. When it rushes you, just strafe away from it. Then use your auto-lock to target it again and resume firing, and repeat if necessary. When you see the bird-bot appear, send your server bots to "collect" it. They will pick it up and throw it into the truck. This counts as an "other" thing collected .When sold, each bird gives you 500 zenny and they always return.

Go to the level where you have to find Diana's tear and keep battling all the Reverbots. Go back to the Gesselshaft, return, and keep collecting the zenny from when the Reverbots were killed. Note: When in that level you have to get the bazooka to knock down the cracked walls

In the Docks level, tell your Servbot to go to the water. When he returns, he will have either a Fish (200 zenny), a Shell (400 zenny), or a Scallop (2,000 zenny). You can tell him to go to a door and he can come back with either money or a Lunch Box.

Land mines
Give a Brain Cube to the Servbot in the torture area. He will gain the ability to set land mines on your enemies.