On any map with shield doors make a big enough box so that a mongoose can be inside (but more like a mongoose and a half).

Go to forge mode and spawn (or pick up one) and place it in the box, then go in there, ride it and run into one of the shields.

It should knock you back into another one, if it doesn't then make the shields closer together, but if it does then you should continue to keep hitting one and playing mongoose ping pong!

It's fun with 2-4 mongooses in there because if things get too hectic they will explode!