you can’t one shot Magic+ Tier enemies even with Crushing Flames (8).
and there was a recent Patch that causes Crushing Flames (6) to lower Normal (Grey) enemies HP to 1 instead of instant kill, because of how it was being used with Frozen Set to get big damage kills in monster packs. since it leaves only 1 HP, it will still look like an instant kill, but just can’t be used with Frozen. from what I understand, you just need to use Crushing Flames (5) and Frozen Set as a combo now.
great idea for your Amulet. I have made that mistake lots of times, and end up spending even more Crystals than I originally intended.