Not half as difficult as the TA ratio would make you think. I went through on Normal, with Fog, Famine, Catch, Thunderstorm and Tough Luck on and finished in 21 minutes and a score of 50,000. So playing on Normal or Heroic with just a couple of these skulls on should be enough as long as you don't die more than once or twice and build up some decent killstreaks.
I always made sure I had either a Carbine or battle rifle wherever possible, along with a plasma pistol to take down shields. The moment there is a ghost available (or the abandoned banshee towards the end of the level), hop in it, use it to take out all nearby enemies, then destroy any abandoned vehicles, turrets, and eventually the vehicle you're driving as well when it's of no more use, this will add hundreds of points each time.
Other than that, this may take you a couple of tries to memorise the level and where to go quickly, but this is certainty a lot easier than some other Par scores