The best approach I've found is to stay in the middle and just fire at the wisps. They seem to mostly spawn in the corners if you do this, which makes for easy aiming. You can ignore the ice blocks until they start to push you around. Not sure if the difficulty escalates, or if it's mostly just about being unlucky with spawns as time goes to infinity, because I've only played it a couple of times and I didn't pay close attention.
Best score: 1773.
Also, it's amazing what you can do for sprites with so few pixels. You're good at this.
