Compa
Skeletron
It always spawns at the very edge of the left side naturally. If it's not there, you'll need to beat calamitas and use an item she drops to create a new one. (it can take a bit like the tool tip says so don't worry if your game locks up for a bit)Where can i find the calamity biome? I heard it was on the far sides of the underworld but i cant see it
Also, any idea why cryogen keeps de-spawning on me?
As for the second question, stay in the snow biome. If you leave it cryogen despawns very easily. So you might want to expand your snow biome if there isn't enough room for a proper fight. Also flying too high can count as not the snow biome, so take care to keep him in view if your flying very high.