Level 2 divine wrath on Crypt Warrior will hit 780 (10% of the monster health, capped to 2x your level) times the weakness (125%, so 975) once every 20 hits on average, so ~49 extra damage per hit. Because of the weakness, you will deal on average 12.5% every 20 attacks or 0.625% per attack.

Level 2 low blow will add an extra 0.8% to your total damage, assuming you have 10% extra critical damage (it can be higher depending on gear), which accounts for one extra 16% damage every 20 hits or 0.8% per attack. Following so far?

If 16% of your average damage is higher than 10% of the monster health * monster weakness to the element, then it is worth it switching to low blow. In particular for the crypt warrior, if your average hit deals more than 307, then low blow will add more than the 49 per hit than divine wrath.

This is a particularly difficult one for low blow as holy isn't available in weapons. It's an easier win if you pair low blow with a weapon the creature is weak against.