Mikkoku features an extensive elemental system for its spells. Each element is strong against different opponents. Depending on what your opponents' resistances and weaknesses are, you can either deal twice the damage, normal damage, halve the damage, or no damage at all. Sometimes, your opponent can also absorb a particular element, meaning the spell will heal them instead. Although this can vary from monster to monster, there are some general thumb rules that can help you choosing which elemental skill you should use in battle. Of course, using your common sense always works well too. For instance, it doesn't take a genius to figure out that you can really hurt a tree with fire magic and a magma dragon with water magic!
There are 10 different elemental properties an offensive spell can have. This doesn't mean that the other 5 elements (Time, Space, Ore, Slime and Dragon) don't have any spells associated with them - they surely do!
Fire is very effective against ice-elemental monsters, as well as against undeads and plants. In addition, it is also often strong against bugs and most animals, especially mammals. To sum up, fire is often a good element to use, with many monsters being weak to it. Just don't use it against water-elemental monsters!
Ice, like Fire, is another element with many uses. Most humans and demi-humans, as well as reptilians and most other animals are weak to it. Ice often works well against plant-type and dragon-type enemies as well. However, it is mostly useless against fire.
Thunder is highly effective against mechanical, bug, and water-elemental opponents. Thunder's main advantage is that there are only a few monsters resistant to it, so you can almost always use it.
Water is mainly strong against fire-elemental and earth-elemental monsters. If this sounds disappointing, you should know that both of these groups are quite common. Plants, however, absorb water.
Earth can be used against almost anything, with the exception of flying monsters, which can't be hit with it. Earth magic is especially strong against big or heavy monsters. Just like water, earth doesn't work very well against plant-type enemies.
Wind, much like Earth, can be used against almost anything. In contrast with earth, wind works very well against flying and small monsters, but not against earth-elemental or heavy monsters. To avoid confusion, it is important to keep in mind that flying enemies are *not* wind-elemental by default.
Grass is a somewhat rare element, with only a few opponents capable of casting grass-elemental spells. It is especially strong against both water and earth-based monsters. Although the spells are slightly weaker than those of other elements, grass-elemental spells have a chance to inflict poison, making it a quite strong element.
Ghost is yet another rare element, mainly used by strong ghost-type monsters. Ghost magic has the distinction of ignoring enemy defense and for working against almost everything.
Holy is arguably the strongest element in the game. Whenever an opponent can clearly be associated with the darkness, holy-elemental spells will deal twice the damage. Since this is, naturally, often the case, you will find that holy-elemental spells can come in handy quite often! Of course, you shouldn't be using this element in holy-elemental places like churches or temples.
Dark is Holy's counterpart. Since by definition, holy-elemental creatures are unlikely to be your opponents, there are only a few circumstances in which dark-elemental magic is really useful. By the same logic, you will regularly come across opponents using this element against you. On the positive side, 'neutral' creatures (foes not clearly associated with either holy or dark) are usually still susceptible to dark damage.