In Fieldrunners (iPhone) what is a good strategy for the Marathon Challenge in Grasslands?

As the question says, I can't get above LVL 33 and I'm aligning 7 Teslas in the middle of the field in a line but it doesn't seem to work right.


Here are two strategy guides intended to be used for Classic/Extended gameplay on hard but which are applicable to Marathon Mode:

http://iphonestrategygames.pbworks.com/Fieldrunners+-+Grassland+-+Extended+-+Hard http://www.iphonestrategygames.com/strategies/fieldrunners-grasslands

Good luck!