Bonus Monster #1:
Fae Reaver
(Introduced in this EnWorld article.)
In a way, that entire article is already a designer's note for the fae reaver, so please be sure to read it. I like the way this monster turned out; it's a serious danger! The only viable options for killing it are area/explosive attacks or strong fighters with incredibly high skill. Archers are probably at the biggest disadvantage, but that's why a wide range of combat styles is necessary in a party. Your best bet is to get a cleric to hold it off via Turning while you prepare an appropriate assault, though the fae reaver is smart and will likely flee to attack another day.