Great job, I've been really in the mood for a game exactly like this.
This is now my default go-to game, and I've been playing it every day since it came out.
Now if only I could figure out how to actually use a strategy other than HOLD DOWN SHOOT AND GO LEFT AND RIGHT, haha :p
One part of the strategy is to use your portals wisely. They regenerate your health, so open them as soon as possible, but only try to go thru them when you get lower than 3 hearts. Staying in the same level as long as possible is usually easier than jumping into the next level which will start slightly more challenging.