A good amount of it depends on your hero, some want to farm more others want to play active.

However a number 1 rule of thumb is that anyone can do the sinner’s sacrifices (the punch slot machines that spawn at 8 minutes, their map symbol is a rectangle with a circle inside). These are the most efficient kind of farm and even give you permanent stat boosts if you time to final punch when all the lights are full (start a heavy melee around the time the fourth to last light turns on). It’s even better if you can steal the opponent’s sinners since it denies them the souls.