In Chantelise, I'm trying to figure out if it's worth it to use up one of my equipment slots with an elemental crystal when I could be stacking up another weapon or defense item instead. How much extra damage will I inflict on an enemy with their elemental weakness? Is this is a constant percentage or does it vary from enemy to enemy?
Elemental attacks deal strictly around double damage compared to normal attacks of the same strength. So for example, on normal attacks with absolutely no equipment, you will deal maybe 9 damage to a caterpillar. With the Wind Crystal, you'll deal 18-20 damage per hit. You can tell the enemy's element based on the color of your targeting circle.
However, keep in mind, the elemental crystals come with their own special benefits. For example, the Wind Crystal changes your combo into a 5-hit instead of a 3-hit, while the Water Crystal gives you a round-about spinattack when charged up (hold the attack button). Mastering the extra attacks is key and often gives you a greater advantage than pure stats do as their damage is not the same as a standard combo. As well, since you can only equip a certain number of each kind of equipment, elements give you even more damage on top of maxed attack equipment (should you go all out).
Also keep in mind that elemental damage allows you to defeat skeletons and other special enemies using just your weapon, not requiring the use of actual spells.