3D Printed Terrain
For terrain I highly recommend Greeblecity. Below is one collection and a few city ruins. We primarily play with 1/400 scale or ~2-3″ models. These sculpts are wonderfully put together and can create a nice looking city terrain for your games. For 1/400 scale we printed the below terrain at 200% scale so the buildings are a consistent scale with the Mechs. Conveniently a 200% scale also fits on a Monoprice Mini printer.
Here are our favorite trees
You can find more free designs from the sculptor below:
Hand Made Terrain
If you prefer to build your own terrain I highly recommend the beautiful terrain work done at Ironhands.com which also hosts the MSEra rules for Gundam tabletop. You can find a direct link to the terrain page here: http://ironhands.com/gundamTerr.htm