I remember watching the Endless Eight arc, I spent like five hours on the second episode because it confused the fuck out of me and I was like "I think I watched this episode" and then I realized something was different and I felt really stupid...

Same

Aren't there 8 eps? or is that referring to the episode in which nothing changes but their clothes and a few bits of dialogue and a scene involving Yuki saying she has lived that same day over 150 000 times?

I remember there being 8 episodes too (I painfully watched them all).
So I guess the achievement would be referring to those other 6 episodes that never changed.
I kinda feel OCD about the achievement being 99G instead of 100G

I cant believe I watched 5 episodes of this endless 8. I just gave up after I realized nothing would change. SO MUCH TORTURE!

I feel that the endless eight was worth it because it set up the plot for the Disappearance of Haruhi Suzumiya which was great.