As with many archer games, pull back and let fly your missiles at the advancing hordes. Each wave gets more and more difficult, so choose wisely when it comes to your upgrades. Either better missiles, more energy build-up, better stability, etc... Everything comes into play.
Tips: Build up your energy and your stability first. Having the missile you want to throw at the advancing hordes blow up in your face instead of hitting one of the tanks is really depressing. Also, the shockwave is easy to acquire near the beginning. Set it to blow next to a line of tanks and watch them blow up in line.
One other thing. This game reminds me of bowling sometimes. If you shoot your missile at the right angle, you may get a strike, rather than just hitting that one pin on the left. Of course, it's also easy to be left with a 7-10 split, too. ;-)
Play Momentum Missile Mayhem at armorgames.