each enemy has an element and a physical structure. all of them will look different and shape of body can tell you what slash to use. element can tell you what element of yours to use. if you need a cheatsheet i think i figured it out a bit :D
Basically.. what I figured: Green(earth-use lightning ). Frost(fire) Purple(use Holy) and Metal use lightning. Based on shapes use attack. (Upper wedge- Overhead cleaver, Six pack shit- plummet, side wedge- side attack, no wedge- thrust)