Here is a Major Mayhem Bonus Tips for you. This is for android version and will works with your HTC Evo 4G also.
If you shoot an enemy when the warning reticule is on him but before he’s launched his projectile you will receive a bonus.
Tap restart instead of re-spawn to take you back to the start of the level to try again to get that top score.
I'm playing Major Mayhem android version on my HTC EVO 4G.