A lot of good discussion again.
Waller is definitely the attribute I have had the most trouble with on my Monk. Merely annoying to face with my Demon Hunter and Wizard, but a constant source of frustration with the Monk. Waller with Desecrator or Arcane hurts. It also seems very stupid that Waller is more powerful on Champions than it is on Rares, even though it does not look as though it were designed that way (smaller wall, less tactical use; coincidentally more annoying). I often start a Waller battle off being trapped between multiple walls, or inside a wall, but immediately focus on spreading the group, interrupting their timers so they do not all cast at the same time, and moving to somewhere with more room (assuming this is possible). I do not think one of these groups has killed my Monk yet, but they consistently poke at my limits.
I agree with Frag: if you are one-shotted by Arcane Sentry, your gear is wrong. My Demon Hunter can take a hit from one of them in Act II/Inferno and keep fighting, so your Monk sure as heck should be able to! I also agree with Frag (mostly) about unkillable Elites. Theoretically, there is no way known you could defeat a Hoard/Extra Health/Fast/Invulnerable Minions pack in a cave or sewer zone, but I have never seen that either
I have never once seen a group I could not kill, unless I was not equipped to handle it. I
have seen plenty of groups that would not have been fun to fight against and quit the game over (usually involving Invulnerable Minions), but not something I felt was 100% unkillable. Now, when two Elite packs spawn next to each other, I have seen
situations that looked unwinnable, but neither pack individually was unkillable.
On the end-game, I am beginning to view it the similar to how Treesh described. Originally, I saw Inferno as Hell+1. Now, I see it as a largely irrelevant part of the game for farming top-end gear and throwing your character against absurd odds. A fine bit of fun when I want to turn my brain off and do something dull, but the fun in the game definitely ends long before you start Inferno.
May the wind pick up your heels and your sword strike true.