After the End/Mutants

Mutants
Due to the mutants not having an official in-game name, I will provide a name for them instead that describes them as closely as possible.

Basic level
These mutants are the weakest, they are easy to deal with most of the time even if they go in hordes.

Basic (four-legged) Mutant - The first mutant you'll encounter the game, these mutants don't do much damage when they get close to you, and they die to a few shots from the Handgun, they die in one shot from the rest of the weapons, this is the weakest.

Avian Mutant - The second mutant you'll encounter later in the game, these mutants also don't do much damage, they die to a few Handgun shots and they will be killed in one shot from the rest of the weapons, however they are immune to the Freezer power-up as well as environmental hazards such as acid, deactivated acid, and lava.

Moderate level
These mutants are not quite weak, while they can be easy to deal with sometimes, they can be quite dangerous when not taken care of properly.

Zombie - This mutant will be encountered last, but it is considerably weaker compared to the rest of the mutants in Difficult level which you will encounter earlier, the Zombie can tank at least 2 - 4 full blown Shotgun shots as well as 3 Machinegun Shots however they move incredibly slow, and they will move even slower on environmental hazards, while this may seem like good news on acid and lava, this mutant can get annoying to deal with on levels with deactivated acid as this means they will have extremely slow moment, making them hard to force into making hordes and you'll most likely resort to taking them out one-by-one, they deal moderate damage to your character but you can get out of them easily.

Spider Mutant - The third mutant that will be introduced to you in the game later on, this mutant moves incredibly fast when running straight but takes some time to turn around and change directions, which means you can stall this mutant by circling around it and slowly making your distance, this mutant can die to a full blown single Shotgun shot as well as one single Machinegun shot, and dies on a single shot from the rest of the primary weapons.

Difficult level
These mutants are dangerous as one could easily cut off a good part of your Armor and HP, keep distance from these mutants as much as you can.

Humanoid Axe Mutant - The fourth mutant that you will see later in the game, this mutant is by far the second most dangerous due to its high speed and deals high damage to your character when it gets close, it is also quite tanky as it could take at least 3 - 4 full blown Shotgun shot and at least 4 Machinegun shots, this mutant aren't easy to deal with and environmental hazards will help you really well in killing these, all power-ups especially the Double Damage power-up should make this mutant easier to kill as well.

Detonator Mutant - The fifth mutant you will be introduced to in the game, this is the most dangerous mutant but with proper killing this mutant can also be a blessing in disguise, this mutant deals massive damage to your character especially on base stats which is why this mutant is extremely dangerous in Firing Range and Survival, this deals damage by exploding at you on contact and also kills other Mutants nearby, this mutant isn't quite soft but not as tanky as Humanoid Axe Mutant, it will take at least 2 full blown Shotgun shots and 2 Machinegun shots to kill this mutant, this mutant can also be considered a high risk high reward target due to the fact that if the mutant horde contains multiple Detonator Mutants that are close to each other, all it takes is to make one explode and you'll kill a huge number of mutants in just a few shots, this mutant also pairs well with AOE weapons such as Sonic Cannon and Rocket Launcher because the explosion radius of this mutant helps increase the overall AOE damage your weapons can deal with, this mutant is immune to Panic Emitter which means while all Mutants will run away from you, this Mutant will still approach you, this mutant is also immune to Contaminator as it will still explode at you on contact if you approach it with this power-up active, if a Detonator Mutant is at your tail and you suddenly activate the Invisibility power-up, approaching a Detonator Mutant that is chasing you earlier will still explode on contact so kill it first, any other Detonator Mutants are safe to approach with Invisibility as long as they didn't chase you, and once again environmental hazards can be used to protect yourself from this mutant by making them explode while also killing other Mutants nearby without you doing anything.

Special
Spawner Mutant - This mutant is blue-colored and holds a staff, it isn't quite common but not rare either as it appears in a few select levels, this mutant can spawn up to 4 or 5 mutants over time and it can be killed with a high damage weapon such Rocket Launcher, it is weak to Plasmagun due to this mutant usually hiding between walls, and it will take at least 3 Plasmagun shots to kill this Mutant.