You can see the name above enemies. You need to kill robots, undead, soldiers will have a regular name above them (frank, sheen for example) and pirates (also have a regular name, like mike)
I got the achievement for killing every enemy just by playing level 48, so level 48 does spawn every unique enemy. However, if you want to get this achievement out of the way, try playing the following levels for each type of enemy:
Soldiers Level 37-38
Robots level 14
Pirates level 6
Undead level 19-20-21