pray that you don't get hit and don't die. Not much else you can do. Killing a series of planes without dying gives you a points multiplier. Use a cluster bomb plane xD.
They really should have this info available on the game itself, but I know that combos and blowing planes up in the air (rather than letting them burn and hit the ground) increases your score a lot.
The easiest planes to get hit are the really fast moving ones, the really slow moving ones, or the laser plane (since the huge laser prevents you from seeing incoming fire). That's why I always use the medium-speed planes with good armor. Ironically it's the laser planes that give you the highest score since the laser hits continuously, giving you continuous score bonus. The moment you get hit by anything the score multiplier stops, not when you die. Therefore it's really hard to keep the multiplier; I think the highest one I've ever got was x8, destroying 8 planes without getting hit once.
T_T it's hard enough for me to finish 12 waves without using all of my lives. I stick with laser planes. homing missiles and cluster bombs are useless for me.
