HL Ryhdon vs. Medi EX

[Tadreth]

If I have a HL Ryhdon active, and my opponent has a Medicham EX active. My opponent uses pure power to attack my bench, will Ryhdon's power stop the damage?

and

If I had Rhydon up against a Spinning Tail Tyranitar, my opponent uses spinning tail to attack my Ryhdon and my bench, my Ryhdon gets knocked out, would it's power stop the damage?

 

[bulbasnore]

Q. Does Power Diffusion reduce the number of damage counters placed, or just damage for Benched Pokémon?
A. Power Diffusion just reduces damage, not damage counters for the Benched Pokémon. (EX:Hidden Legends FAQ)

So, Rhydon would not stop Pure Power from placing counters.

On Spinning Tail, I believe the answer is found in the rule book, where we find the last step of the attack process:
Do whatever the attack says. Do any damage first, then do any other effects, and finally, Knock Out any Pokémon that have damage greater than or equal to their Hit Points

So, T-Tar would do all the Spinning Tail damage (Enough to KO Rhydon, but none to the bench with Rhydon active). THEN you check and see that Rhydon is knocked out!