Sorry all. I stuffed up. The problem would have been fixed by simply making blue gems worth a lot less... and hence the score would have been fought by how many guys you could let loose and kill with rocks.
Unfortunately when setting about 500 levels up its hard not to see techniques that will make a level trivial. It's hard enough to ensure all levels are possible in both single player and teamwork mode.
Still, it took me a while to work out how you did it.... there's always a few secret or hidden methods left for different people to discover. Pity this one was really easy to work out but I didn't see it.