share the save (quicksave right in front of him, use winzip to pack up save_14 folder, upload to sendspace and share the link).

also, to anyone who is experiencing the same issue (enemies who cannot be harmed by weapons) - SHARE THE SAVES. it's the only way how to hunt down this rare bug, and maybe fix it somehow.

Will do first chance tomorrow.

Solved
(for me at least)

Once the mob was chasing me I quick-saved, then quick-loaded. I was then able to shoot and kill the Hybrid.

My rep must be too low so I can't post any links for the quick save file.

just add some spaces to the address and it should work (like sendspace file/fryfu1), or send it to me via private message. that broken save (if you still have it) is priceless, we have been trying to hunt down this bug forever, but the bloody thing is just so rare.

www dot sendspace dot com /file/ 2aix06

There you go.
After you load it just keep heading forward down the passage. The buggy mob will be around the first left corner.
Hope it helps.

EDIT: I did some tests after I posted this. After every load of the save I seem to randomly get one of the following results:
1. One Hybrid is waiting around the corner and is invincible.
2. One Hybrid is waiting around the corner and is killable.
3. Two Hybrids are waiting around the corner and only one is invincible
4. Two Hybrids are waiting around the corner and and both are killable.