Like most RPGs, Unto the Breach uses an elemental system that can lead to enhanced or reduced damage when utilized (im)properly. However, its elemental arrangement is a bit different from most, so let's take a look at how it all works!
The Five Elements
While most RPGs use some variation of the tradition earth/wind/water/fire elements, Unto the Breach makes use of the Chinese Five Elements, which are Fire, Earth, Metal, Water, and Wood. These elements exist in a cycle of feeding and growth, as outlined in the picture to the right.
Nearly every monster you fight (except for fellow humans and a couple others) belongs to one of the five elements. Each element will have a different effect on the target, depending on the relationship between them. The monster's native element will have no effect on them, while the element which feeds it will heal it (e.g. Fire feeds Earth). The element fed by the monster's element will do normal damage. Additionally, each element "insults" one, and "destroys" another, indicated by the straight lines on the chart. Each element destroys the element two places down, and insults the one three places down (e.g. Fire destroys Metal and insults Water). Attacking an insulted element will do +50% damage, and attacking a destroyed element will do +100% damage.
Let's take a Fire monster for example:
There's a great deal of interaction between each of the elements here, but proper utilization will let you do tremendous amounts of damage to most monsters. Every one of the four characters in Episode One utilizes them in some way, especially Lucy, whose damage output is almost entirely element-based. Ali can also unlock skills that further decrease a monster's resistances, allowing for even nastier combos!
The player characters have no built-in weaknesses or resistances to any elements, but certain pieces of equipment or unlockable talents can grant you some resistances, reducing the damage taken when attacked by that given element.
In addition to the Five Elements, there are three other elements that appear in Unto the Breach. Holy and Dark are diametrically opposed elements to each other, each drawing upon the raw energy of another plane of existence. As no monsters will attack you with Holy spells, you don't need to worry about resistance against it. Likewise, you can't use any Dark magic, so there's no monster resistance against it.
However, you can still build up resistance against Dark magic that some monsters use, and Holy attacks will do extra damage against certain kinds of monsters (particularly undead animated by Dark energies, and demonic entities from the Great Enemy's home dimension). Holy attacks also have the benefit of being able to damage everything you fight, it just won't do a great deal of damage against monsters not explicitly weak against it.
Finally, there's Arcane, representing the raw potential of humanity and our own universe. Only a few abilities will deal Arcane damage, but it has the advantage of being equally effective against all potential targets, as nothing has innate resistances against it. Its base damage output is also higher than Holy, making it more versatile at the cost of not being able to do massive burst damage against anything weak to it. Only human opponents will have access to Arcane attacks, but building resistance to it with the player characters is still possible.