Ran fine for me. I also found it easy, got up to about level 7 so far without dying.
I'm adding more levels, probably about 30 or 40 in total, which is why the first ten here seem so easy. Trust me, after the first boss, they'll get a lot harder. Here's level 12...

I think maybe it is too easy because the gameplay feels a little slow. Maybe try bumping up the speed of the game in general.
I'll add levels of difficulty so that people can choose which speed to play at.
Also maybe you could give a small window of vertical movement instead of locking it completely.
I actually had that a first but decided to remove it so that the player could concentrate on charging the enemies. Later on in the game smaller enemies will interfere with the beam constantly. Hopefully there are more things I can add to make it more of a challenge.
There's also the fact that the most common enemies are essentially passive targets, though that helps introduce the "charge and detonate" gameplay in a relatively "safe" environment.
Heh...not for long.