This is the type of question people made 7 years ago...
Are you talking about tentacles in the game or in one of your maps?
If you mean the game, here's the walkthrough for that HL level
http://www.planethalflife.com/half-life/guide/walkthrough/wt5.shtm.If you are playing Opposing force, I think you're not meant to kill any of the tentacles.
Finally, if what you want is to kill the tentacles in your map, then I have never tried it...