It was a fine test. It was a hard encounter test.
No, you can't say that with a straight face.
So here we have a situation that is considered a typical hard encounter with PCs needing an 18, 19, or 20 to hit without other situational bonuses.
No, we have a situation that is the HARDEST hard encounter going against one of if not the HIGHEST defense of said creature, AND you're tying the party's hands by not counting ANY additional to-hit bonuses from feats, race, powers etc. Calling that typical is absolutely ludicrous.
To illustrate your "a party should have an easy time getting those to-hits under a 10" claim, you would have to state exactly which bonuses that you are talking about.
How about a rogue, going against reflex who took demigod and sly flourish and has combat advantage, which, despite what you say, is trivial to get, there is a 11 point difference right there, taking it under 10 to-hit WITHOUT any other powers or benefits of the party like triumphant strike, marks and the like. And again, remember, this is the HARDEST hard encounter, NOT, as you claim "typical."
Maybe this is why I've never seen ANYONE who has actually played in epic say it's anything like hard to hit. Even the hardest possible +4 encounter is still pretty easy to handle before you start piling on all the benefits of stuff from Adventurer's Vault, Martial Power and PHB2 and the like.