Besieger

Lord of the Rings Example
     DM: "The oliphaunt continues rampaging along as the archers seated atop it continue to make attacks. You can try to climb up and unseat them with Athletics or Acrobatics."
     Legolas: "Excellent! I'll start with a couple of Athletics checks to scale the beast as it runs by. 26, and 32."
     DM: "The first one fails, and you can't quite get a grip, but then your hands latch on to an arrow sticking out of the oliphaunt's side and you hoist yourself up."
     Legolas: "Okay, some more Athletics, then. I climb the rest of the way up so I can cut the ropes holding the archery platform in place. 29, damn, 17, and 31."
     DM: "You steady yourself atop the moving oliphaunt and move to cut the ropes as the archers panic. I'll need a couple of Acrobatics checks to make sure you don't fall off too early and can safely dismount."
     Legolas: "Cool, cool, my Acrobatics isn't bad. 28, and... yes! 34!"
     DM: "Great! You slice the rope holding the archers' platform in place and it slides off, and you do a quick tuck and roll to avoid breaking your neck on the way down like they did."

Prerequisites:

Besieger Elite Artillery
Defenses +2 AC; +1 Fortitude, +2 Reflex
Saving Throws +2
Action Points 1
HP +6 per level + Constitution score
Traits
Secret Weakness
Choose a set of 2 or 3 skills appropriate for tackling or analyzing the besieger. For example, Religion and Insight for a ghostly mage, or Thievery and Acrobatics for a clockwork siege platform. An adjacent opponent can engage in a skill challenge to discover or expose the besieger's weakness. DC 20 + 1/2 level, Complexity 2 (6 successes before 3 failures). Success means that the besieger can no longer use the Endless Power ability. Failure triggers the Knockback power (see below.)
Minor Actions
Endless Power (minor; recharge 4)
The besieger regains the use of an expended encounter power.
Triggered Actions
Knockback (immediate reaction when an adjacent opponent fails the Secret Weakness skill challenge; at-will)
The monster makes a basic attack against the opponent who failed the skill challenge. On a hit, the target is pushed 2 squares.