Warthog joyride
Whizz around in a Warthog, letting your passengers pepper the enemy. Maybe do a bit of squashing too! It'll take quite a while to whittle them down but I'm pretty sure you'll enjoy it. Ideally your seated passenger would be a sniper; it would be nice to have Sergeant Johnson from the Pelican area as he has great vocals. You could also have a sergeant on the chain-gun; could be Stacker, or even another Johnson! See Extra marines for some relevant info.
Difficulty levels
For the most fun with this plan I'd suggest using Normal or Easy so you can withstand more fire. On Heroic you have to be rather careful not to expose yourself to the mob for long. It's risky driving into their midst, and you tend to have to repeatedly speed away to recharge (plasma balls from Jackals are a particular problem). If you've got Marines they're likely to be killed off pretty quickly in the meanwhile. At present almost all my twin bridges megabattle saves are for Heroic, but I plan to make some for Normal and Easy sometime to see how much better this plan plays (and likewise the Banshee defence).
Woo-hoo!
There's a jump available for a bit of extra fun. When coming down from the far end on an anticlockwise circuit, look for the rise in the ground near the end. It's not very obvious, but it's a big enough jump to make your Marines whoop - and to cause you to overturn if you're not careful!