If you have some Planetstrike Crash & Burn markers (God I love the look of 'em) and some other bits and bobs you could slightly adapt one of the Battle for Macragge, I remember them starting small and then getting a bit bigger. I learnt with a cityfight at Warhammer World (On their amazing Tyranid/CoD board), I was Orks he was Marines, heavily balanced in my favour of course, but the objective was to capture the crashed shuttle. I won it as it's in a Cities of Death Ruin, upon which I deployed my Lootas and in which I deployed my 20 Sluggas. He had merely 10 Marines, just ran 'em forward blasting his bolters at my sluggaz (in cover, so he hadn't a hope in hell of killing them all) before sitting on the objective, within assault range, now down to 5 men from the Lootaz. The sluggaz charged, and obviously won. Great game (No, not just because I won)!
Midnight