Does your monster have to be rank D? My Tiger/Abyss with over 600 strength somehow lost to him in arm wrestling. >.> The only think I can think of is that he was rank A, but I don't see why having a higher rank monster would exclude you from it.

I believe it is a percent thing (like a 10% chance of loss?), so maybe you just got unlucky?

90% chance of success. The only requirement for you monster is that it has a POW of at least 90. It has to be a minimum of rank D only because you need to have Rank D to do that Errantry.