What has four legs, sharp teeth, and feasts on human blood?
It's a pair of medics from TF2.
One medic übers the other, who stabs people with his übersaw to charge his über meter. By the time the invincibility runs out, the second medic should be fully charged and able to make the first one invincible, who then charges his meter using the saw, repeating the process until there are no more enemies around. This method is totally silly, and can be devastatingly effective. It is also a lot of fun.