You can unlock this on any level with a Salamander 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 1000 rice. Note which color is your primary profile and with one of its units haunt the Salamander Shrine which is at the bottom center of the map next to the cliff with two other shrines on it. This will spawn a Salamander Monk on the following turn. Kill three of your secondary profile's units and have the Salamander Monk eat their skulls so that he becomes a Demon Salamander Monk. On your secondary profile's turn, group five of your units (not counting your general) close together, and then end your turn. Select the Demon Salamander Monk and move within range of the group of five units, choose Spell, and select Lightning which will be the spell all the way to the right. This will damage all of the units. End your turn and immediately end the turn of your secondary profile as well. Cast the Lightning spell again, the units will all die, and the achievement will unlock.
I recommend doing this achievement at the same time as "Epic Win" to save yourself time killing all the units that have to be created for that achievement. Note that doing so requires creating a pass and play or online game, rather than local as detailed at the beginning of the guide for this achievement.