my biggest tip is double ship as soon as possible and rapid fire!!!
 Kill as many as you can before they form, and if you cant kill them all before they form then concentrate your firepower on one side of the formation that way when they begin to swoop down the bullets are only coming in one direction. rinse and repeat, the difficulty peaks at around level 30-40 and remains more or less the same up to about round 100. then it becomes just a test of endurance and reclaiming a double ship as soon as possible once you get hit..... When it gets past level 30 its almost bullet hell type dodging to get the double back but its very possible.
Once you get the patterns down you really can play this game until your hand falls off, thats normally around level 100 for me
. thats when my RSI tells me to stop!
And get used to playing with the dips on hard too, as its a bit of a jump when you go from normal to hard!