Space Giraffe

Basically, learn how to 'Bull' for maximum points - Make sure you have at least one jump pod so you are able to jump (necessary and easy to forget in the heat of the moment, you'll be stuck having to use your superzapper otherwise and will be kicking yourself) then lurk at one end of the grid (if it's a flat grid) and wait until there are as many beasties as possible crawling along the top of the grid towards you. On a circular grid, simply follow them round the grid, watching out for flowers and bullets as you go. They all move at the same speed so this is easy enough to do. The early levels at least...

When you've got as many lined up as possible, you will find you have no Power Zone left and are very VERY vulnerable - you can't 'bull' them out of the way like this but, no problem, just use one of your jump pods to jump (and come down in same spot, don't go jumping over them shooting as you pass) which will instantly fill your powerzone - NOW bull them all off the grid in one fell swoop to the right (or left, depending on which end of the grid you were I suppose)

Do this and once all the bonus points have finished adding up and the cacophony of mooing has died down you should find you have a 9x multiplier, or very close to it (if close, just do it again and you'll have that multiplier in no time) - this method makes it much easier to get your 9x multiplier and keep it, thus making it fairly easy to keep it for five consecutive levels. Also makes it easier to get the higher scores.

Note I say "easier", not "easy". I don't profess to be an expert, just passing on the technique that helped get me this achievement...