Neat game! I really like the swapping time mechanic and think you could do some more cool stuff with it.
I agree that the enemies could ramp up quicker, but I'd definitely want an automatic weapon/hold to fire. My strategy was to go down the lower hallway and shoot blindly up, then watch the points roll in. Adding an objective that requires the player move around the level or having the coins need to be picked up might help force them into a more aggressive play-style.
Nice job!