The phalanx of enemy invaders moves laterally across a grid not much wider than itself. When it reaches the edge of the grid, the whole army lowers a notch. Rule one: narrow that phalanx. Before you do anything else, take out at least three enemy columns either on the left-hand side or the right (for Waves 1 and 2, the left is recommended). Thereafter the aliens will take much longer to cross their grid and slip down another rung. Keep on working from the sides: you’ll find that the invaders take forever to trudge and shuffle back and forth, and you can pick them off in your own sweet time.