Humanoids
Feral Ghouls
Feral Ghouls are humans that have been incredibly irradiated and somehow survived. The process of transformation has sent them insane. If they notice a human they emit a high-pitched moan and attack.
Strategy
Feral Ghoul - Two headshots from most guns, or a solid hit from a melee weapon will take them down easily, but keep your distance if you are a low level.
Feral Ghoul Roamer - The Roamer is a lot faster, does more damage, and takes more damage than the regular ghoul. The best way to deal with these is to cripple the legs and attack the head from a distance.
Feral Ghoul Reaver - Reavers are very, very fast and powerful so crippling their legs is paramount. They are also armored so the head will take most damage.
Glowing One - Glowing Ones have a dangerous radiation attack they use if you get too close, so keep your distance. Cripple the legs then go for the head.
Feral Trooper Ghoul - Trooper Ghouls are like Roamers, except they sometimes attack with knives. Use the Roamer strategy.
Glowing Trooper Ghoul - These are armored Glowing Ones and are really dangerous. Stay well away, cripple its legs, and as always go for the head.
Vault Security Officer - These are the same as Glowing Trooper Ghouls. Use the same strategy.
Vault 34 Overseer - The Overseer is very difficult. He has very high HP and the same attacks as the Glowing Ones. You should deal with the turrets in his room first, as they deal lots of damage, then cripple the Overseer's legs and arms, then focus all your attacks on his head.
Super Mutants
Once human, now horribly mutated. Super Mutants are by and large insane, but retain speech and are able to use firearms and melee weapons.
Strategies
Super Mutant - Super Mutants are thick-skinned and coming armed with a variety of weapons, so they are a challenge. If they wield a melee weapon, you should cripple their legs to slow their advance, while if they hold a gun, target the gun in VATS to make them drop it. Then just deal as much damage as possible, as they take the same damage everywhere.
Nightkin - Nightkin are Super Mutants that can turn invisible. They should be dealt with similarly, with the proviso that if they turn invisible, you need to attack them outside of VATS to make them targetable.
Insects
Bloatfly
Bloatflies are a mutated insect that travels in groups of four or five.
Strategies
Bloatfly - The Bloatfly is one of the weakest enemies in the game. They generally won't attack unless you get close, and do so by firing small projectiles at you. They do weave around in the air, so it's easiest to kill them in VATS.
Cazador
Cazadors are mutated insects, possibly evolved from paper wasps. They tend to appear in swarms. Try and stay away from them if you are a low level. Bring Antivenom.
Strategies
Young Cazadors - These are the weakest kind of Cazador. They don't do a great deal of damage individually, but tend to come in groups and you can get overwhelmed quickly. By crippling the wings, you can slow them down enormously and make them easier to hit. They should go down without too much hassle.
Cazadors - Dangerous. Cazadors are very fast, have high endurance and have a fast-acting poison. They also come in groups. If you come across them, always cripple the wings of every member of the group first. Once the wings are crippled, they will walk towards you at a much slower rate, and mines are effective. They can take a lot of punishment.
Legendary Cazador - There is only one Legendary Cazador. It is much larger than the others, has twice as much health, and a great deal of attack power. You should cripple its wings to make it a lot slower. Again, once crippled, mines are effective, but be careful as there is not a lot of room for maneuvre.
Fire Ant
Fire Ants are Giant Ants that breathe fire.
Strategies
Worker Fire Ant - Worker Fire Ants are fairly well armored, but susceptible to attacks to their heads and antennae. If you cripple their antennae, they will become Frenzied and attack anything close by. Keep your distance to stay out of range of their fire breathe.
Soldier Fire Ant - Soldiers are the same as the workers, except double the size and power. Keep out of their range, try to cripple their antennae and focus your attacks on the head.
Giant Ant
These are ants that have mutated to somehow more than 10 times the size. They tend to come in groups of two or three.
Strategies
Giant Worker Ant - Slightly armored, but easily frenzied if you cripple their antennae, so they can be turned against each other. Attack the head.
Giant Soldier Ant - Bigger than the Worker, more heavily armored and packs more of a punch. Cripple the antennae then go for the head. If you have a lot of HP or Stimpaks, they are weak to melee attacks.
Giant Ant Queen - There is one Giant Ant Queen. She is at a relatively high level and has a lot of defense, so kill any other ants nearby and focus your attacks on her head.
Giant Mantis
Giant Mantis are evolved from the Preying Mantis. They tend to live in places with plantlife.
Strategies
Giant Mantis Nymphs - You can step on these to kill them. They are the weakest creature in the game.
Giant Mantis - They can deal a reasonable amount of damage, but have low defense, so almost any weapon will be adequate to kill them.
Giant Mantis Female - The same as the regular Giant Mantis, just a bit more powerful. Being insects, they are vulnerable to being frenzied when you cripple their antennae.