But I think, I have finally come up with the ultimate fieldrunners strategy for the grasslands map:

The key idea of this strategy is to channel all “land” units along the top first, getting heavily bombarded by the “mortar towers”. This enables a tighter design of the “lightning towers” in the middle.

I just played it through once, getting to 299, but I think there is still room for improvements / optimizations of the general approach, so someone should be able to break the 300 some time soon.

Here is my “building plan” for the strategy:

…please let me know when you made any improvements to it

Update: Rick kindly agreed to send me a screenshot of his insane round 479 strategy, so here it is: