A Clr 9 has 8+8D8 hp without Con bonus. That's 44 average. To kill, he neds to do 54 damage.

A Masterwork Greatsword (Magic Weapons can't be Shattered, like Xykon does to that very sword) deals 4D6+Strx3.0+4 (assumes Greater Weapon Spec). Average is 14 for 4D6. So, Strength needs to do 36 damage. That makes his Str bonus 12, for a Strength of 34. That can be achieved by some, but I find it very hard to believe in OotS world where magic items are few and limited in potency. even with a max damage crit of 28+Strx3, we're looking at a Str bonus of 9, for Str 28. Still unlikely, though barely achievable with a +6 Str item and 4/5 of level attributes put in Str.

Additionally, there's the fact Roy actually comments on it. If it was just a Crit, Roy would just have thought it was a normal roll of 19-20, and wouldn't have commented.