You want to farm this ideally in the 1st level.
Blow one of the outside walls up and take your time dispatching the various enemies with your shovel. I recommend going into one of the corners and spam the attack button as they all walk into you.
This is the way to gain extra health/shields and the devil marks you need to legit complete the game(You spawn the devil on 5 marks)
The game tallies your points at the end of a level or death so don't panic if it doesn't pop when you hit 3,6,12k points.
You can see me earn all 3 points achievements within 7 mins of grinding the outer walls in the video below if you need it.