Entertaining for a while.
If you do decide to make it so you can fire in any direction, then you'll have to re-think the gameplay a bit.
As it is, it's enjoyable enough but I don't like the way the enemy moves.
To be able to keep going you need to be accurate and not waste your bullets. With the combination of the time limit and the random movement, that can sometimes be impossible...
I think this could be a great game when it's finished. You need to maybe just put a little more thought into the time limits and the movement of the enemy.