Quickly build your way to a monolith, then concentrate on a another one. You must keep up the pace due to the increasing speed of enemies with each wave. Each monolith gives you a special boost. Start with the speed monolith to speed up your minions. Then, build the monolith that slows down your enemies. Follow that with the monolith that offers a boost to attack range. Finally, choose the extractor bonus monolith.