Back to Him

What you see when you activate the ability is its channel, not a cast time. You must be actively channelling the ability when Athanos finishes his cast of Power Stomp. Click the ability immediately before Power Stomp finishes, as your block is instant cast and only lasts briefly.

I also had a hard time at first. This is what worked for me.
Hit your extra-action button when the bar reaches the last "P" in Power StomP.

When I first started this quest, I thought this quest was severely bugged. I spent a bit of time frustrated because I kept getting hit by the Power Stomp even though I had activated the Humility's Guard. It took a few minutes before I realized that the Humility's Guard spell is a channeled spell, and thus figuring out what I did wrong.

What this simply means you have to have Humility's guard channeling as he hits. Here's an example of what I mean:

THIS IS THE CORRECT WAY TO HANDLE THE STOMP

Power Stomp is casting
Cast Humility's Guard with 0.4 seconds or less remaining on the cast. You want Humility's Guard to be channeling WHILE the stomp hits.

I made the mistake of thinking you need to just press the action button and have the buff up before the low lands, like this example:

THIS ISN'T THE CORRECT WAY TO HANDLE THE STOMP

Power stomp is casting
Immediately casted Humility's Guard to pre-emptively have the buff on
The channel finishes before the stomp hits resulted in an unsucessful block.

So a slight TLDR on this; Make sure you are channeling the buff Humility's Guard AS THE POWER STOMP IS HITTING YOU, so basically cast it with 0.4-0.1 seconds remaining on the cast.

I hope this make sense and clears up the frustration for those who didn't finish it yet and are looking for a solution to do it without wasting so much time from constant misses.