Remathilis
Legend
Our goal here is to generate grind-less encounters that are out of the adventurer's reach - without resorting to what many 4E defendants always propose here, namely not staging the fight at all.
"The dragon wakes up and breathes fire. You die."
...
What are we talking here as "out of the adventure's reach"? 5 levels over the PCs? 10?
Here's a simple [non-fiat] method.
A level 4 encounter is 875 XP. A level 9 elite is 800 XP. Therefore a level 9 Elite is roughly a level 4 encounter (if your anal, add a level 7 minion to make it truly 875) (Recall also that according to the DMG, a PC group can face one encounter 4 levels higher than them also, meaning this is a difficult but plausible challenge for 1st level PCs, as per the DMG).
A Bullete is a level 9 elite. It has 204 Hp, AC 27, Fortitude 26, Reflex 21, Will 21. Its basic attack has a +14 to hit for 2d6+7 (4d6+7 vs. prone), and it can burrow underneath foes (knocking them prone) and spray rocks in burst 2 for 1d6+7 damage. Oh, it can 2nd wind for 52 hp.
Compare that to a typical level 4 PC. 32-54 hp (depending on class and con), AC 17-23, +7 with implements, +10 with accurate weapons. The bullete can knock the PC prone as a move, and then use his standard to do 11-31 damage to that foe. That'll down your wizard quickly. And since the fighter needs a 17 or higher to hit and has no way to really do 254 hp to it, I wager it takes 6-8 rounds to TPK the party.
All from a level 4 encounter! So you want to make challenging encounter without the grind? Figure out what the max XP of an level-aprop encounter is, then use 1 monster of that value. (A level 13 encounter is the same XP as our bullete, I suggest a vrock.) I bet the PCs don't last 10 rounds!