http://divinity.wikia.com/wiki/Divinity:_Original_Sin_CysealGo to R on this map. You'll find 4 statues here, each for a different element. Use an element on the corresponding statue to spawn an elemental you have to fight. Spells, grenades, and arrows all count as elemental attacks. Assuming default camera settings...
Southeast - Fire.
Southwest - Water (ice work too)
Northeast - Air (lightning work too)
Northwest - Earth? (poison works too)
If you aren't sure something works, check the description and see what element damage it does. Note you need to hit the actual statue, not the button looking thing under the statue. After hitting all 4 with the proper element and destroying all of the elementals, you'll get this achievement.
If you go inside the cave you'll find an old friend from Divinity 2. I'm not sure if talking to him here if required for the achievement related to him, but I'd do so anyway.