Easiest way to get this in less than 10 minutes:
- Select first Mission - Osiris
- Normal difficulty
- Now activate the following skulls: Catch, Fog, Famine and Thunderstorm.
Those skulls are easy and won't give you a handicap, they will just multiply your score.
I also used the Cowbell, Grunt birthday party and Iwhbyd skull but I don't know if they add up to the multiplier bonus, (someone confirm please), if you have those I suggest you select them anyway, they can't hurt.
Now, beating the par time is easy since it's on normal difficulty, just make sure you kill all the enemies and try to get multi kills - that will increase your score.