Spiked Ice Slime

Nothing summons it. It appears on its own anywhere cold, mostly in the caverns beneath a snow biome, and it keeps trickling in after hardmode at a much lower rate. Newer versions also let it use loose ice and snow blocks placed outside the biome, so a frozen floor you laid down for looks elsewhere can quietly start producing them.

The projectiles do not hit hard. Every one that connects chills you, and so does touching the slime, and being chilled on ice already means your sidestep turns into a long slide into whatever else lives down there. A smaller share of hits freezes you outright and locks you in place until it wears off. Expert stacks a second freeze roll on top of the base one, so the hit you shrugged off in a classic world is the one that kills you in an expert world.

In classic it lobs one spike at a time on a slow timer and you can walk between the shots. Expert swaps that for a burst that sprays out all around the slime, so standing next to it is the worst option available and a short sword is the wrong tool. Kill it at distance. Royal Gel skips the fight entirely and turns it passive, which is worth a slot if you are only crossing the snow.