Kroton Soldiers are enemies that are hard to beat. Their theme is marked with a K. If you beat them, you will not get them in your lab. Luckily, their rarity is always 1, and have a low level usually. They appear in Every location. Nothing gets a +1 die against it other than the rarity and level.
Elite Krotons are quite similar to Kroton soldiers. Their rarity is 11 (gulp!). They have a rather red armor. They are in Diver's Demise, in Zoctopot trench. The first one has a level of 16. They usually are in the last level in every location.