Most users report that Red Vein strains work best to mitigate pain.
My personal experience is that Red Kali and Red Dragon are not the strongest for pain and are closer to green strains by effect and feel.
Red Maeng Da (pimp grade) is one of the stronger ones for pain. If one tolerates Kratom very well the Tinctures could very well be a better option for pain.
I will not use the term pain killer as one cannot kill nor eliminate severe nor chronic pain. The point is to make things bearable.
Best of luck to those seeking relief.