I've significantly updated the game and it's available here (I DELETED the old one):
http://www.newgrounds.com/portal/view/
559544

NEW FEATURES:

CLOAKING IS NOW AN ARMOUR ABILITY:
Drawing inspiration from halo. A meter on the right of your HUD show's how much power you have when engaging this ability. Once the power depletes to zero, you'll need to wait
till it recharges to a certain level (icon will glow) before you can engage it again

DIFFICULTY CURVE SIGNIFICANTLY CHANGED:
Users complained that the game was too hard to begin with and they would get swarmed by enemies within the first 5 minutes of gameplay. To address this I've:

* Added a more gradual difficulty curve with a system of rounds and waves:
Each round enemies become more agressive, more varied, more numerous and will sometimes have upgraded speed and combat capabilities.
Currently 3 waves make up a round (but that will change when I add more enemies in the future)

* Limited the number of enemies that can stalk you around the map:
This feature took over a week of programming (on/off) to get right and prevents players from getting swarmed by a ridiculous number of enemies on the field.

This limit increases each round.

NEW CREDITS SYSTEM:
Credits are earned by destroying enemies.

Turrets ports can now be unlocked by spending Credits at them:
-400 Credits to unlock a turret port
-50 Credits to Respawn a turret

MISCELLANEOUS:

* Fixed glitch with enemies standing stupid if they stand where you were 3 seconds ago.

* Optimized code for custom controls
Remember that if the fire & strafe keys are the same for 1st/2nd player,
than the controls in Co-Op defaults to:
L: Strafe (1st player) Shift: Strafe (2nd player)
Enter: Fire (1st player) Space: Shoot (2nd Player)