Slime

Slimes are a common creature found throughout the world of Slime Taoshite 300-nen, Shiranai Uchi ni Level Max ni Nattemashita. Despite being relatively weak and dispensing little EXP, they are the main reason as to Azusa's maxed out level, having been killed by her in the dozens daily for 300 years.

Background
Slimes have existed in the world long before Azusa reincarnated into it. Beyond that, not much else is known.

Characteristics
Slimes are small, roughly the size of a human head, appearing to be alike single cellular organisms and are predominantly blue in color; slimes of a lighter hue of blue are filled with evil and liable to attack others unprovoked, while those of a deeper blue are peaceful. Often of a low level, they are incapable of dispensing enough damage to kill a similarly levelled Azusa and as such do not provide a great deal of EXP to constitute a level, however it is possible to farm them for long enough to max out your level. When killed, slimes drop a crystal that can be exchanged for a modest sum of money, and a day's worth of casual farming of these crystals is enough to support a self-sufficient lifestyle. In certain circumstances, slimes are capable of developing a higher intelligence and building their level. Most of the time this results in a somewhat stronger slime, but it's possible for slimes to progress further than this. The most principal example of this is Azusa's twin daughters Falfa and Shalsha, who are the culmination of 250 years of slain slimes by her hand, and as such are significantly more advanced than other slimes, possessing the ability to reconstitute their forms into that of humans that are as convincing as the real thing as well as being capable of learning to a high degree, including a Smite Evil spell targeted towards the Witch of the Highlands.

However, this is not the only method of slimes becoming more powerful and attaining human forms, as another slime known as Wizly managed to become adept enough at magic to construct a transformation spell to manage this, and similarly another by the name of Fighsly improved her strength stat such that she could physically reform herself into a human.

Trivia

 * Slimes are a common enemy appearing in many roleplaying games, from table top roleplay to JRPGs. Despite their reputation regarding this medium, one of the species earliest appearances is in H.P. Lovecraft's At the Mountain of Madness from 1931 in the form of the Shoggoths, a race of amorphous protoplasm originally engineered by the Elder Things.
 * As seen with Shalsha and Falfa. If a slime spirit born of slimes killed by one person attain a human form, the take on the gender and looks of the person in question; they had Azusa's face and hair (minus the color difference).