Many RPGs use an Advantage / Disadvantage System for their battles and while this offers the strategical and tactical depth that an RPG needs, it also delivers problems that need to be solved by the designer.
Different types of systems
The first question that will occur is: “What kind of advantages and disadvantages will my game feature”.
One of the basic systems is the Element System with water, fire, wind and earth.
Water will do more damage against fire, fire will do more damage against wind and so on.
This system works well with a small number of elements, because players will almost instantly understand that the increased damage is coming from an advantage against the enemy’s element.
How different elements behave towards each other is widely known outside of games and can therefore be used without offering a big tutorial or learning curve. However, increasing this system will lead to cross advantages that will add complexity to the system.
Suddenly fire is not only strong against wind, but also against nature and bug - while being weak against water, ice, rock and metal. It gives more strategical playground, but at the price of understandability. Players might get confused, where the difference between water and ice is or fire and magma. With every element having multiple advantages and disadvantages, the chance that the player makes mistakes (even late in the game) due to the number of elements he has to consider, increases drastically. Does an increase in elements in this system not further enhance the system itself or offer more strategical depth, it only adds complexity.
Another common system is the Weapon Type System. Here specific weapons deal more damage against specific enemies. Sharp swords for example do little damage against armored enemies, but increased damage against leather type enemies.
The basics of this system are similar to the Element System, only with weapons instead of elements, but also slightly harder to understand for the player. While everybody knows that water extinguishes fire, the proper counter to a sharp weapon might not immediately be known.
This system requires more time for the player to learn to use it properly. Additionally, increasing this system will lead to confusion pretty fast. While players might understand the difference between a sword, a hammer and a dagger the difference between a spear and a halberd might already be harder to grasp.
The last system I want to bring up is the Status Effect System with curses, buffs, poison and others. While it can be argued if this system even fits into this category or not, it offers an advantage or disadvantage over the enemy. Lowering the defense of a heavy armor type with a curse, can have the same effect as using a heavy sword against him.
The Element System and Weapon Type System are offering advantages / disadvantages against every enemy and character all the time, while the Status Effect System creates an advantage only for a period of time against a specific enemy, usually without offering the disadvantage.
For example, decreasing the defense of the heavy armor increases the damage of sharp weapons (disadvantage turned into a normal state), but since the armor type is not changing the heavy weapon will still deal more damage (advantage still advantage).
The problem of mixing systems
“My game uses weapons and magic (elemental and status effects) so, I will combine those three mechanics to offer the player a combat system with maximum depth.”
While the idea behind this sentence is good the result will probably be very different.
All those systems are basically the same. No matter if it is a fire spell against a water enemy or a sharp sword against a leather armor, when the system is stripped down to the mechanics it means for example 50% more damage against the weakness. Therefor bringing in all 3 systems is not increasing the depth but only the complexity of the game.
These systems will not add new layers on top of each other, but increase in size horizontally.
The player is asked to remember all 3 systems and all the advantages and disadvantages.
Since implementing more and more weapons or elements is easy, the size of each system will grow and grow. Eventually the player likes one system in particular and will only focus on using that, because he doesn´t want to be bothered with learning all the others.
He chooses for example the Weapon Type System and becomes a professional in choosing the right weapon against the right armor type. The other two systems are being ignored, because doing 50% more damage or 75% more damage is not offering another strategical level. He might do a bit more damage against an enemy, but at the cost of more time spent learning the system than playing the game, so the outcome does not justify the effort for the regular player.
It caters to min/max players but not to the broad audience. The goal of offering more depth to the combat system is therefore not met and since the designers wanted to use all three systems the chances are high that at one point the player will face a specific enemy that is designed to use all three systems. At that point the player gets frustrated and thinks the game is unfair, because of this powerful foe. He might have forgotten parts of the other systems or tried them out once in a bad situation and failed, which led to a bad experience with the system and him never using the system again.
The question is, why does the game need multiple systems? And does the combination offer more depth or complexity? Stripping it down to one system, but really sticking with this system and implementing it in other gameplay mechanics as well (for example puzzle solving or specific areas only to open with the right amount of element types) might offer the game more than five more elements that need to be put somewhere in the advantage / disadvantage chart.
The balance of the advantage
Balancing an RPG is always a hard task. Players might be level 10 or level 20 when reaching a certain point, and the question is always: should enemies scale with the player or should they be fixed in their level and stats? Also, how is the advantage handled in this fight. Is the fight a challenge even when using the right advantage or should it be an easy fight, when choosing the right weapon.
Of course, there is no real answer to those questions since they depend on the experience the designer is creating for the player, but there are things that are just not optimal.
Introducing an Element System for example and offering enemies throughout an area that are water focused is perfectly fine. The player does not only understand the Element System, but also the theme of the specific area. With this in mind, he will face the boss thinking he will face a Water Boss. But what if he suddenly is fighting against a Wind Boss?
That would anger and confuse the player, because he cannot see the reason behind this.
The designer might have thought of tricking the player and offering him a surprise, but that is not the intention of the system. It should rather be clear, so that he can further improve his skills in the Element System and not offer him surprises that he might not be prepared for, thanks to the rest of the level design. And when the boss is not the wrong element, but actually neutral and does not have any advantage or disadvantage at all, the whole system will fall apart.
Smaller enemies throughout a level are used to learn the system and play around with it.
They usually do not offer a challenge and can be defeated without using advantages.
After defeating a bunch of those, the player wants to have a challenge when he reaches the boss and he wants to feel satisfied and clever when figuring out what weakness his enemy has and how to exploit it. When the boss is outside of the Element System with a neutral state, the player is stripped from this satisfaction and leaves frustrated, since all his learning has been for nothing. This effect is especially important when using a Status Effect System.
The whole system is useless when every boss is immune against Status Effects.
The only effect this will have on the player is not using the Status Effect system at all, because in critical times the system will not work anyway, so why waste time on that?
When creating a RPG Battle System with advantages and disadvantages some questions need to be answered:
1. What feels like the right amount of advantages and disadvantages in my system? 2 might be too less but 20 is getting to complex.
2. Why do I want to mix different systems? What is it offering my gameplay?
3. Am I really creating depth or complexity?
4. Is my system useful in critical battles? Will it provide the advantage in stronger fights or can I ignore it?
Answering these questions makes the difference between a boring fight against a neutral type, non-elemental Eater in Digimon Story Cyber Sleuth and an awesome fight against the fake President in Final Fantasy 8 where the player can use the advantage of him being undead, by defeating him with heal and resurrection spells.