There's a reason he is called "The SAINT of All Killers", as opposed to "The Master of Killing" or "The Champion of Killers".
His guns are divinely blessed. They carry the power of God Himself.
Mister Immortal is just a mutant. There's no way that mutated genetic codes can override the will of God.
The Saint of Killers shoot Mister Immortal, he dies and stays dead.
If you want someone who can contend with the guns of the Saint of All Killers, you need someone on God's level, like Q (from "Star Trek") or Pennywise or Eternity. Not a mere mutant.
In a single post, you have managed to show a complete lack of understanding of, not just one, but 6 different characters. Including the Saint of Killers, God from the universe of the SoK, Q, Pennywise and Eternity.