Combat mechanics

There's a lot of nuance that sets Nightmare Reaper apart from the basic "shoot at it until it dies". Most of it applies in some form both to player and to monsters.

Damage multipliers

Headshots

Almost every weapon does 200% damage when aimed to the head, and almost every monster has a head. This multiplier can be increased with upgrades from Jade Tree and Topaz Tree.

Monsters cannot headshot.

Critical hits

Weapons with the "critical chance" stat deal 300% damage on lucky shots. Both the multiplier and the chance can be increased via Jade Tree and Topaz Tree. Increasing chance applies to all weapons, even those without the "critical chance" stat.

Critical damage stacks with headshots. A lucky hit to the head can deal six times the base damage.

Elite enemies can deal critical damage.

Damage types

Most weapons that don't have an inherent elemental effect can be augmented with a weapon stat granting them fire, ice or shock effect. Weapons that do imply a certain element (e.g. Fire Bolt Book) can have a matching effect stat further boosting their effectiveness.

Both elemental resistance and elemental power can be upgraded in Skill Tree and Topaz Tree.

Elite enemies augmented with a certain element only affect their intended target (usually the player).

Fire

Fire is the damage-over-time element. Monsters, scenery and player can all be set on fire, and spread it around with physical contact.

There are many environmental sources of fire, from broken braziers to lava pools.

Water pools extinguish fire. When fighting an enemy that attacks using fire (such as Monks), it's often a good idea to stay either in water or close to it.

Ice

Ice attacks slow monsters and player alike. Both movement speed and attack rate are affected.

In addition to that, while frozen, monsters die at non-zero percentage of health. This is of greatest importance when fighting bosses, as their huge health pools get greatest reduction point-wise.

Electricity

Electricity is the area-of-effect element. When electrified, monsters occasionally discharge lightning into adjacent monsters. Player can't attack or use grappling hook when shocked.

Electricity spreads in water. Every bit of electric damage dealt to water itself or monsters standing in water spreads out far into the adjacent cells. It is advised not to use electric weapons while submerged.

Combined

Monsters can be affected by multiple elemental effects at once. A weapon cannot have multiple elemental effect stats, nor can a weapon with an inherent element have a different elemental stat. However, a weapon can have the "random projectile" stat that replaces its projectile with something of a different element. For example, a Sawed Off, a weapon without an inherent element, can combine shooting Ice Bolt Book pellets with a fire effect stat.

Getting a monster to suffer from all three elements simultaneously earns player the "Storm, Ice and Fire" achievement.

Ballistics

In Nightmare Reaper, firearms are not hitscan, like in most old-school FPS games, but are instead based on fast projectiles. Therefore there is noticeable bullet drop and travel time for the projectiles, requiring player to aim higher and predict target movement when using sniper rifles.

Environmental hazards

Falling damage is brutal. It can easily kill monsters that either get pushed over the edge by their peers, are thrown around by explosions, or miscalculate their jumps. Meanwhile the player can negate falling damage in a variety of ways, including, but not limited to, double jump, grappling hook, Chained Saw altfire, Skill Tree upgrades, and more.

Traps of many forms can be activated by timer, touch or proximity. While most traps are relegated to challenge rooms, there are some to be found in general fighting areas, where they can be used to player's advantage against monsters.

Pools of various harmful substances are a common sight in nightmares. Like low oxygen areas, they make routing as much of a priority as aiming and shooting. Jade Tree has upgrades to both negate damage from liquids and extend the amount of time the patient can hold her breath.

Death effects

Monster deaths can be a lottery on their own. If a monster dies while on fire, it might turn into a running charred corpse, spreading fire until it collapses for good. Sometimes a monster dying from electricity will spend its last moments in intense convulsions like a makeshift Tesla coil, electrifying other monsters in vicinity. A monster killed with a sharp weapon will usually fall apart like a cardboard sliced in half (although this doesn't have any gameplay consequences).

For a monster killed by any other weapon, e.g. by firearms, it's possible to enter a "bleeding out" state. In that state, monster is given a random time to vomit blood with repeating pain sounds. If player manages to finish the monster before its internal timer runs out, they are rewarded with a shower of coins.

Elite enemies always drop a weapon, a big life soul and a pile of treasure when killed. Elite enemies of "bomb" type also explode like a nuke (without the toxic waste) in addition to that.

Infighting

Whenever monsters accidentally hit each other, infighting ensues. The advantages of this are numerous: ammo economy, free pickups, reduced danger to player as attention gets divided, not to mention plain fun of seeing nightmare tear itself apart. Even monsters of the same type can infight in Nightmare Reaper (unlike e.g. in Doom).

Stained weapons

Any player's weapon can get covered in blood of enemies. This effect is purely cosmetic. The weapons can be wiped clean by switching, or washed by entering water, waterfalls or interacting with sinks.