Every monster is defined by a few main characteristics: their role in battle, their relative strength, and their basic nature.
Monster roles are the greatest determinant of a monster's abilities, how they fight, and how they should be fought. Different classes will have different advantages against monster roles. Monster roles also affect how monsters interact in group dynamics, and differing monster roles creates variablility in the encounter dynamics. The concept of monster roles is drawn most explicitly from DnD 4e, though it exists in a variety of systems in some form or another.
Artillery: Artillery are the primary enemies which use ranged attacks. The majority of their attacks deal simple damage, though they have more abilities than average that can deal damage to multiple characters. They typically have lower defenses than other creatures, making up for this through movement. Damage is typically average and often consistent. Defeating artillery is best accomplished by closing ranks and meleeing.
Brute: Brutes are very simple enemies, but that is not the same as easy. They have few abilities, their abilities typically only deal damage, and their strategy is often just BASH. They do not have very high defense or good dodge, but they have the highest hitpoints. They also have higher damage and are often able to critical, though their accuracy is low. Brutes can perform actions to increase their damage and are willing to make sacrifices to deal even more. When a hero faces a brute, he should try to manipulate it so it represents the least threat possible.
Controller: Controllers are the enemy type with the greatest range of attacks. As the name implies, controllers focus on controlling the battlefield and forcing their opponents to make bad choices. Controllers limit a hero's options with disadvantages. Snares and roots limit melee, wounds limit healing, silence limits spells, and stuns limit everything. Controllers also use a variety of bleeds, area effects, and damage fields. Controllers are typically ranged. While they can deal high damage, typically with long cast time abilities, they have low defenses and health, and are not always consistent. A hero facing a controller typically benefits from killing them quickly, but even more important is not losing track of them. As long as the hero is aware of the controller's actions, he can react intelligently.
Lurker: The key aspect to lurkers is spike damage. Typically undetectable at the beginning of battle, when they first hit, they hit hard. They have low defenses and health, so if detected, or after they strike, they can be rather quickly eliminated. Their greatest defensive strengths are dodge and cowardice. If they maanage to escape, they may be able to hide again and repeat the process. Some will even get back their strong abilities after a period without hiding. While many use simple health attacks, others might give disadvantages such as bleeds: anything to give their side an advantage from the get go.
Medic: Medics are defined by their ability to restore the health of their allies and remove their disadvantages. While rarely featuring strong attacks of their own, they can often improve their allies. They feature moderate defenses and health and can withstand a sizeable amount of damage if they are healing themselves. Furthermore, they are often more resistant to disadvantages. Players facing medics have one main decision: Does killing the medic first kill the other enemies faster?
Skirmisher: Skirmishers focus on movement as their greatest advantage. They close quickly with heroes, are able to follow after if the hero runs, and if things go poorly, they themselves can run. They have less defense than soldiers, fewer hitpoints than brutes, and fewer options than controllers, but they are versatile. They are the most likely enemy to have reactions. They typically have a ranged ability or two and are likely to hit with their attacks. They can also keep a character guessing with their actions and have no significant weaknesses. In many ways, skirmishers are the average enemy, and a hero should fight them based on their companions.
Soldier: Soldiers are the most defensive enemy and also the most consistent. Beyond high defense and average dodge, soldiers typically have the ability to parry or block attacks, lowering the damage they take even further. Soldiers often have active abilities that can reduce the damage they take and react to attacks, and the very best can protect others. Because of this, a hero must be careful when and how they attack soldiers and their allies.. Fortunately, while soldiers have high accuracy and are consistent with their damage, they lack variety in their attacks and have lower damage. Heroes that deal high damage in single attacks are stronger against soldiers, and many heroes can afford to wait to kill soldiers last.
Minion: Minions are the weakest strength of enemy a hero will face. They are meant to be fought in hordes and one good, solid blow will knock them down. They have few abilities, but they do work well in groups. Typically, they form homogenous groups alongside standard or elite monsters. Due to their high quantity and low health, area attacks are often the best way to deal with them.
Standard: Most enemies in the world belong to this strength. While certainly one alone is danger to a hero, a group of three or four can prove to be a challenge. Often times, standard enemies group together in varying roles, each serving a purpose to the larger group.
Elite: Elite enemies are the most typical leaders of enemy groups a hero will encounter. They are difficult, but a hero can overcome them solo without any major fear of death. With strategy or luck, a hero can defeat a pair of them, or one commanding a couple standard enemies or minions. Elite enemies are common in areas where heroes venture forth in parties. Elite enemies typically have abilities that enhance their allies.
Lieutenant: Lieutenants serve two major purposes. First they replace elite enemies in challenging areas meant for parties of heroes. Second they are often the commanders in events or quests. In the role of commander, they typically stay out of battle until the hero has defeated groups of other enemies. In this idea, they are self sufficient, with many abilities and complex tactics. Still learning how they think and fight can earn a player sn advantage.
Boss: Fighting a boss is always a significant occurrence. A hero needs both strategy and luck to emerge victorious. Often times boss fights involve multiple stages, each requiring different tactics. Bosses may also involve their servants either as part or between stages. Thankfully, because boss fights are significant events, heroes may find advantages in their environment, such as allies or weapons.
Monster nature is a mostly thematic and artistic element, though it does have a few functional aspects. Creatures of the same nature are likely to have similar strengths and weaknesses. Further, some items and equipment will consider monster nature for bonuses or penalties.
Aberrant: Not necessarily a catch-all category, aberrants do include those monsters that are alien from any other nature and from each other. Often times, aberrant monsters originate, in story, from a different place from the natural world or hell. They do not share the same common physiology of other creatures, and have desire and motivations which are difficult to understand.
Elemental: Elemental monsters are primal and associated with at least one of the four basic elements. While elemental monsters include the obvious animated elements, they are include creatures intrinsically attached to elements such as dragons and genie. Elemental monsters have strengths regarding their own elements and weaknesses from any opposing elements.
Humanoid: Humanoid covers a broad range of enemies. Humanoids compose the four great kingdoms, and independent groups in between. Humanoids are capable of doing almost anything, and often remain versatile while doing so. Humans very in power from the weak who will serve anything to those strong enough to command dragons and demons. Humanoids include all the basic races of the world and a variety of other sentient species.
Natural: The natural category contains two major subcategories: plants and fairies. Plant creatures include treants, choker vines, and cursed trees, and are sometimes limited in their movement. Fairy creatures include the common sylphs, pookah, and goblins. Natural monsters have two typical motivations: protect nature, violently if necessary or even easier, and to serve the cause of the Sylvan Queen. While the Sylvan Queen is currently absent from the world, the creatures often still serve her.
Vermin: Spider, insects, and rats, vermin are the lowest of creatures. Closely related to, perhaps technically, animals, vermin are mindless carriers of disease. Bleeds and wounds are common forms of attack, and they often have defenses against disadvantages. Vermin are often minion class, but they still have their queens and champions.
"Tell me about the rabbits, George."
"Shhh. You don't want to know about them. They are one of the most dangerous monsters to exist. Half again as tall as a man, they're at least twice as broad, and it's all muscle under their white fur. Some men call them wild hares, but they call themselves Lagum. The only thing sharper than their axes are their cruel incisors, and they will leap leap at the enemies with both. They can hear a child's heartbeat with their long swept back ears, and their twitching pink noses can smell fear. You can tell the strength of a Lagum warrior by the head swinging from his belt: the fresher and hairier, the stronger they are. You don't talk about them, lest you invite them around, and with them, the Frost."
(Written by GLHargrave/JCLance)