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
Powers
Endless Power (minor; recharge 4)
The besieger regains the use of an expended encounter power.
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.)
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.