Monsters
There are various monsters in the game, all of them encountered in the mines:
Flies
Danger Level
Low
Behaviour
Simply flying up and down a fixed path
Strategies
Simply hit them from the side.
Drops
Maggots
Danger Level
Low/Med - They are very slow and deal small amounts of damage - Danger lies in higher numbers of them at the same time. READ THE BEHAVIOUR!
Behaviour
They will come in (small) groups and attack on sight, although not in a group intelligent way. If you manage it wisely, you can trigger them one by one. (And you should!)
If they take too much damage, they will quickly run away from you and pupate, to turn into #Moths. During pupation, they turn blue and negate any damage dealt to them.
Strategies
Pull them one by one, moving close enough to aggro one but not many others. Use a weapon with as little knockback as possible, since you don't want to throw them into the rest of the group or give them the chance to run away. Kill them quickly before they can run away.
Drops
Moths
Danger Level
High - They are fast and deal moderate amount of damage.
Behaviour
They will fly at you at high speeds and deal drive-by-damage.
Strategies
Sword timing - Stand still and swing at them right before they would hit you. Use a weapon with high knockback to make it easier.
Drops
Stone Crabs
Danger Level
Low
Behaviour
They disguise themselves as stones, sometimes surprising the player.
Strategies
Either use the pickaxe to knock the stone off their heads, which makes them flee. Kill them while they are fleeing.
Or preferably: Use your weapon to hit them directly.