Par score: 18,000
This is a pretty easy one to get, considering you're in a tank and it's easy to tack up sprees and other high-value kills like vehicles. It is very likely you'll get this through normal gameplay on Legendary, but if not here's a great video guide.
Keep in mind this video was done on Legendary with Famine, Mythic, Thunderstorm and Tough Luck. The skulls are not entirely necessary, but with these you will absolutely destroy the par score.
One last thing to be aware of is once you pass the par time (nine minutes), your score does begin to decrease and your score multiplier begins to drop as well. Despite this you shouldn't have trouble getting this one finished.