Additionally it's worth to remember that half of "red" damage is applied to stun damage (not visible but it's there). Of course that means applying only red damage will kill opponent.
Myself I never used stun baton on phase 2, instead shock weapons/first phase stun rockets/alien weapons in addition to applying red damage first. Preferably with shotguns as shotgun will also apply stun damage. You can pretty much stun opponents with shotgun, not totally as it will kill unless pure stun damage is applied. But still, shotgun red+blue damage first (or just red damage) and then pure stun after it works fine. Just using pure stun is painful unless you have same phase equipment.