I also had issues with the last ones. This helped: http://www.youtube.com/watch?v=HUTr-Sx1veE
Also the power converter makes a huge difference. I use like this, with more powers to blasters while i got shields (u see shield status o the far left, its the blue around ur ship). If my shields are completelly gone i either go shield mode for a few secs or hit the ability where enemies cant target me. Depends on if u have to attack at that time or not. If u dont already, practice on making evasive manouvers while firing. See the comments on the tube.
Edit: Theres one more thing. Maybe ur having problems with the slightly harder fighters. SThey looka abit bigger. These needs to die fast, cuz they hurt u bad.