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.