What is the safest & fastest way for ending a pentazosine addiction?

There isn't any: fast way. At least not to end the ADDICTION, which is something that requires long-term psychotherapy, counseling, peer support and changing the very way we think and act. The dependency can be treated more quickly, and pentazocine withdrawal is generally not as severe as full agonists. A short taper of pentazocine itself, or with buprenorphine or methadone (usually low doses) is adequate.