You can unlock this on any level with a Fox Shrine, in any game mode. The easiest method is to create a local game with two players on the Chaos Canyon map, with unlimited time and 100 rice. Note which color is your primary profile and with one of its units haunt the Fox Shrine which is at the top center of the map. This will spawn a Fox Monk on the following turn. Kill three of your secondary profile's units and have the Fox Monk eat their skulls so that he becomes a Demon Fox Monk. On your secondary profile's turn, kill one of your primary profile's units, and then end your turn. Select the Demon Fox and move within range of the skull left behind by your deceased unit, choose Spell, and select Revive which will be the spell all the way to the right. Confirm the spell and the unit will revive and the achievement will unlock.
This can easily be done in the same session as "Were You Using That?"