Exploiting Weakness in D&D 5e: A Guide for the Evil Plotting GM

I wrote a similar article to this on Pathfinder, and though this is a pretty evil topic, it has been of use to a lot of folks out there.  So, now, I think, is the time to start a similar article for D&D 5e.  This will be a living document to be updated with suggestions and more information as I find better methods.

In general, 5e is a lot more challenging for a GM, because the classes are reasonably balanced and because the game mechanics don't have a lot of scale over the 20 levels.

Here's a general list of things to increase the difficulty for the party:

• Increase the number of foes and the CR (ignore the book CR system if it isn't offering a challenge)
• Spells against the PCs low saves.  A high level sleep spell, for example, could take out a party with no elves.
• Drag out the opportunity for the long rest.  After 2 short rests, the 3rd battle of the day is tough.
• Hit the most vulnerable PC first. It almost always forces another teammate to take an action to assist.
• Use terrain to force the casters in close to get line of sight.
• Make the foes so they can quickly switch between ranged and melee.
• Place skill checks in the middle of combat to pull off key PCs.  For example, a trap in front of foes with longbows pulls the rogue out of combat.
• Use your high AC / damage resistant foes to "soak up" all the PCs' reactions early in the round so the rest of the foes can move around at will.
• Place your foes behind terrain and obstacles so the melee fighters can't reach them while they do ranged attacks.
• In dungeons, a 5' wide tunnel with a turn is ideal.
• A simple 5' door is a death trap if you have polearm-optimized foes in the room, waiting for them.
• A slick muddy steep hill with longbow wielding foes on top is usually a serious threat since it may require Dex and Str to get up it (climb plus not slipping)
• Just imply ranged attackers separated by a rivine is a big obstacle.
• Hidden attackers can also use terrain to surprise melee attackers whent hey do try to advance.

Exploiting D&D 5e Classes
Class	How to Exploit
Barbarian	Barbarians have DR while raging that makes them near impenetrable.  The key is to either stop their raging or hit them with damage outside their DR. Barbarians lose their rage if they haven't attacked or been damaged since their last turn, so back off and let the rage fail before engaging.  An illusion that draws them away but disappears before they attack is ideal. Monsters outside their DR that do psychic damage include Aboleth, Banshee, Shadow Demon, Fomorian, Intellect Devourer, Mindflayer, Succubus, Incubus,  . Spells that do psychic damage include Dissonant Whispers, Dream, Feeblemind, Geas, Phantasmal Force, Phantasmal Killer, Staggering Smite, Vicious Mockery, Weird, Wrathful Smite, .
Bard	Bards can be a little bit all over the map, but usually they have low enough AC and HP that they don't want to get into melee.  They are also using
Cleric
Druid	When in animal form, these guys usually drop to a lower AC for HP.  You need to do massive damage, and you can use a lowered attack bonus to get there.  Feats like Great Weapon Master and Sharpshooter on the foe side of the table give you the edge to inflict the required damage.
Fighter	The most important thing you can do to disable a fighter is to make them spend their action doing something else.  Set them on fire so they use their action to put out the flames.  Hit them with a spell, so they are doing saves.  When they get their actions with multiple attacks, there is little that can stop them.
Monk	I've never seen an effective monk that may the player happy they were playing a monk.  That being said, they are often squishy and don't do well against high AC targets.
Paladin	The bane of the paladin is a High AC foe.  If they don't land their attacks, their abilities just aren't effective.
Rogue	Deny sneak attack by giving him disadvantage.  Terrain is good for this.
Ranger	Two weapon rangers get showered with ranged attacks from a distance. Ranged attack rangers get engaged in melee, preferably by hidden foes.  Add high winds to give them disadvantage on ranged attacks. Rangers with an animal companion get their animal companion attack to draw them out.  Then attack per ranger rules above.
Sorcerer	See caster.  See wizard, ignoring spell book.  Try to get them to use up their limited points early, so they can't use them later.
Warlock	See caster.  Warlocks have very limited spell slots, so getting them used up is key.  Avoiding short rests is good too, since short rests are key to warlocks.  Warlocks often multiclass, which may give them sparse stats across the two classes.  Use these lower ability scores against them.  Usually warlocks are weak, so grappling is an idea way to ruin their day.
Wizard	See caster.  One of the strengths of wizards in flexibility in spell preparation, so don't give an indication in advance as to what type of spells will be needed, i.e. changing damage immunities. The spellbook is flammable.  If they don't have a backup, it is a prime target for fire spells and for thieves to steal.
Melee Combatants	Lots of ranged attackers, especially behind difficult terrain, can render these guys completely ineffective.
Ranged Combatants	Hit them with melee to give them disadvantage on ranged attacks.
Casters	If they have a concentration spell up, hit them with some damage, even minimal to force them to make a spell concentration check. Engage in melee.  Their low AC, low HP, and disadvantage on ranged spell attacks makes this the most dangerous situation for them. Force them to spend their spell slots early on utility things so they don't have them in combat.