"Assault on *insert name*'s keep" -Mordheim Scenario
This little scenario can be stretched out into its own mini-campaign depending on what you want to do. First I label one player the attacker and the other the defender. You can stretch it out by doing a few battles before the climax, like: Scouting scenarios where one player controls a scout team hunting down a few people, followed by a normal battle, then to the actual assault part.

For terrain I would create a type of castle or fort either stretching the length of the tabel (frontal assualt only) or in the middle of the table. The attacker gets to choose his forces as normal and my position himself anywhere as long as he is atleast 30 inches from the castle/fort's wall. The defender starts the battle with half of the attackers total number of models either inside or outside the castle walls. The attacker's objective is to storm the walls or smash down the door and get atleast one model inside the keep. Before the battle begins, give the door an amount of wounds agreed upon by both players. To smash the door, the attcker must "attack" the door and lower it to 0 wounds.