Check out Zodiac for an example. The Cancer level has a Gleeok in the upper right, and the Cancer corridors spawn them occasionally.
It appears that Gleeok heads do not die when their health is reduced to zero by a script. This probably has something to do with how the Gleeok is coded. Scripted weapons can damage the Gleeok's body directly, and THIS can kill it. But if the Gleeok's head is low on life, only a blow from a hard coded weapon (not a scripted one) seems to knock off the head or slay the dragon.
How exactly are Gleeoks coded, anyway? They appear to be multiple enemies.
They *are* multiple enemies. A 4-headed Gleeok is actually 5 enemies. Each head has 1000 hp. Every frame, the body looks at all the heads. If any of them have less than 1000 hp, then the body subtracts the difference from its own hp (the real hp of the Gleeok) and resets the head's hp to 1000. When the hp drops below a certain amount, the Gleeok body disconnects the last head in the enemy list that is currently attached. When the body's hp reaches 0, all of the heads from the Gleeok are deleted and then the body is deleted.
To tell a head to disconnect, set its misc variable to -1. When you are ready to kill the Gleeok completely, set its body's hp to -1000. That will clean up all of the heads.