So the mage decides to shoot the rock asunder with a spell. How do you quickly adjudicate that in the 4E skill challenge structure? There simply is no easy way to structure all the possible actions a PC can use in such a situation. There is basically two responses:
A: Toss the skill challenge as written overboard: "Roll a spell attack" - Despite the rules that skill challenges use skills and not attack rolls, and that the DCs appropriate to skills and attack rolls are different.
B: Stick to the rules and punish the players ingenuity: "You fail, get hurt, and now you only have 2 potential failures before disaster strikes"
First, the Mage makes an Arcana-Concentration check to be able to shoot under pressure.
Second, determine the "stats" of the rock, basically, AC and HP. Assign fitting numbers. Is is a very big rock? A very hard rock? A very metal rock? If all you can think of is "it's a rock", it's probably granite. Granite would have moderate AC(it's pretty tough), and low HP(once broke it fragments easily) Lets say it's AC is 15 and it's HP is 10.
Well, does the mage make the shot? If he succeeds, yay! Go mage! If he fails: acrobatics check to jump out of the way of the rock. Fails? Rock makes an "attack" on him. Is it big and rolling downhill? Then it has a high-to-hit(it's hard to miss the only thing in front of you) and high damage(killing-level high). If you want to get really nit-picky, assign the rock a movement speed, multiply it's size category by it's speed to determine "force", multiply by d10's in the same way you do falling damage. So, a rock moving at 10 squares per round with a size category of "large"(1 medium, 2 large, 3 huge, ect..) would be 20d10. That's also sufficiently high enough to ensure no player thinks they're tough enough to just "stand and take it".
So we make a quasi-skill-challenge combat.
Also: did I really just stat out a rock?