There is the 'mikey' trick on the first brain wave. The brains all try and go for him, so if you can protect him and kill everybody but one or 2 enemies. Then you can rack up the points/freemen by collecting all the mommies after.
Try and kill the quarks or whatever they're called before they spawn stuff.
I like to attack the enemies whenever possible, don't run away, but run towards them.
Shoot and move on, don't wait for your bullets to hit something.
I've gotta counteract 2 of these strategies...
1) Don't run towards enemies - it just gets you killed on later levels. Learn to run away from them while shooting back at them. This is true of most levels except Brain levels (see below)
2) Brain Level 101.
Brain levels have a surprising amount of strategy to follow, depending onm what is currently happening.
Brains - do NOT get hung up on protecting the single 'mikey' or equivalent except on the first couple of Brain levels. On later levels it is just impossible.
Instead, at the beginning of a level try to do a quick scan and combo of the following 2 -
collect humans
clear a space around you by working out the least crowded part of the level - do not stay central, go to left or right of screen, not very edges
As soon as there are no more easy humans to go for, concentrate on killing Brains/enfiorcers and avoiding their missiles. The key to later levels is to kill Brains as fast as possible as the homing missiles are the hardest items to dodge on these levels.
Once you have cleared an area around you, shoot all Brains above or below you to try and clear the third or half of the screen you're in. This gives you a bit of time to dodge missiles.
At this stage run up and down the screen aiming at the remaining Brains until too many missiles get close THEN start going clockwise or anticlockwise around the screen, always concentrating on shooting Brains first, other enemies second, and only concentrating on missiles that are just TOO close. (in effect pretend the missiles don't exist except when very close to you)
You HAVE to learn to use your diagonal shooting on these levels.
Also, in general, Brains will shoot missiles if you get too close to them, usually giving you no time to dodge. The very first missile of a level is deadly and seems to be very accurate - DO NOT be next to a brain when the first missile is launched - you will notice a small delay after the level start before they start firing, although this time delay reduces significantly (virtually instant) on higher levels
Also, NEVER EVER stay still for even a second - the homing missiles lock onto you and are much harder to shake or kill.
Of course, all that I have described above takes mere seconds and a lot of practice to perfect. Get it right and you'll lose less lives than you gain - get it wrong on later levels and be prepared to get slaughtered.
3) TANK Levels
The tank/enforcer levels are a whole other strategy, mostly involving being ultra aggresive at killing them ASAP, and going for formed tanks in the order they appear as, like the Brains, you have a few seconds between them forming and when they first fire.
Once again, fast reactions help you, but do not go to the edges (especially the 4 corners) and circle as best as you can, only reversing direction if deemed absolutely necessary.
I hope this helps - these tactics are especially useful on scores over 500K when things start to get very hairy
TS