Official Sega Dreamcast Magazine: Demo Disc 4 Cheats
for Dreamcast

Be Invincible as Kurt in MDK 2 Demo

When Dr. Hawkins is telling you how to fire your weapon, move, etc. get to
the point where he says he placed a gernade somewhere around there. Then
go pick up the gernade but do not throw it. Go to sniper mode and hit the
ball in the doorway that keeps you from getting through it. This will
cause Dr. Hawkins to stop talking and will take away the gernade from your
inventory but it will make Kurt invincible! Well worth it!