The Mountain arm wrestling - no matter how big you are, there’s always someone bigger and badder. Apparently the title World’s Strongest Man is not what it used to be. (Admittedly, with (Europe) behind it, it is a bit limited from the get go.) Maybe Oberyn should have arm-wrestled him.
But then he didn’t lose the fight with his arms but with his mouth, so...
EDIT: Plus here’s an actual scientific breakdown of how to defeat The Mountain. Thanks to mxyzptlk,who seems to be kind of a pro in this... :)