Some interesting strategic play in this one. You have to kill the UFOs from your current loop as your first priority, but otherwise you want to be putting out bullets that will be useful for future loops, clicking the fire button off cooldown in a direction there isn't currently a bullet. (I would have liked a "hold mouse to keep firing" option.) Staying in the center is best for the safety of your current loop since UFOs can spawn anywhere around the edge without warning, but a bullet fired from the center covers less ground in future loops than one fired from an edge across the whole screen.
I didn't ever manage to get past loop 4 or so across a few attempts.