"May I never boast except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world." NIV translation
Unlike the judaizers, Paul simply told people (boasted) of the wonder that, through the cross, he was made right with God. From that time, his mind was focused on the things of God and not the things of this world. That is the sanctification that Peter spoke of in his first epistle.
"Neither circumcision nor uncircumcision means anything; what counts is a new creation. Peace and mercy to all who follow this rule, even to the Israel of God." NIV translation
The Law does not save anyone (even the Jews). Peace comes to all through the Prince of Peace which is Jesus Christ.
Mercy (not getting what we deserve) comes from trusting in Him. This promise is for all men and that includes the Jewish people.
"Finally, let no one cause me trouble, for I bear on my body the marks of Jesus." NIV translation
The marks that Paul refers to are called stigmata and they are scars. Paul had many scars from the scourgings that he had suffered for the gospel. He considered these marks to be sort of a badge of honor as he was considered to be worthy to suffer as Jesus did. The "trouble" that he refers to is not the physical beatings but the attempts at mixing the law with the gospel of grace through faith in Jesus Christ.
"The grace of our Lord Jesus Christ be with your spirit, brothers. Amen." NIV translation
Paul finishes the letter with the hope and prayer that we will see with our spiritual eyes. He is talking about the Spirit of God living in us and bringing us peace.