All the 8 player levels that come with the game are impossible 6 vs 2 with the sliders all the way to hardest! They actually need to come pretty far down for the levels to be possible. This is mostly using the tactic of taking out two bases very early game and then trying to expand with their oil plus ally. In theory, you have half of oil, but you actually seem to fall to far behind in tech while you are busy wrecking bases. Holding out in your own base area is a no-go and dominating oil seems to be not working (though looks like it could on Mizamaze). I say mostly, because on levels like Yin-Yang, the bases are too far spread to take out two enemies.
Full of ideas - Most are probably useless. Feel free to ignore them