Not so much a solution but more of a tip.
I used Cindy 3rd down on the left as she has going missiles,you could also pick the ninja character and his fully upgraded shurikan also home in. The key I found was on the first level there is a set of 3 tanks.....if you hit and collect the first 1 you get 2000 points 4000 for the 2nd and 10000 for the 3rd. You have to kill and collect straight away for this.a little further in the level is another set,same principal applies. Now if you get the sand coloured level with all the tanks then when you shoot the chimney looking enemy a CZ appears.....as with the first level collect these as soon as you destroy them racking up the point. After the 1st level I was on 66k (have had up to 79k with other ships) and 190k by the end of level 2. For the 3rd (bonus level) this is why you need a homing attack ship you collect the power ups giving you 2k per one. I got up to 282k after this.
Hope this helps