Our DM recently made us fight an Elder Brown Dragon. I'm sure it was a level appropriate encounter, but it was a rather difficult fight. The room we fought the dragon in was really tiny. When the dragon used its breath weapon, the only safe places were 1)the 1 square wide hallway we came from or 2)the 1 square wide section on the opposite side of the room where his breath wouldn't fit.

Other annoying things about the battle:
-We had 2 characters who could only attack with ranged powers(seeker and wizard), so they got attacked each time they tried to do anything.
-The previously mentioned wizard specialized in fire spells. Unfortunately, the dragon's fire resistance made it so the wizard only did single digit damage.
-The dragon's special movement, the ranged counter, and the breath weapon left us blinded most of the fight.

We eventually won with no casualties, but the druid was pretty close to dying.

We thought that we were going to have a tough fight against a Dire Bulette last session, especially when we fell 80 feet into a hole and ended up fighting some golems and shapeshifters, but it turned out to be rather easy. Long story short, I used Dismissal to permanently remove the bulette from the plane.