I've seen SpongeBob and Squidward as Survivors but don't know how to unlock them. Am I missing something?

Spongebob can be obtained in the curent event and squidward in past events. They both are exclusive and therefore do not have survivor shards.

Also, use defensive skills such as defender, force barrier and molotov and then some high dmg skill such as destroyer and whistling arrow and soccer ball (it does ridiculous damage and destroys bosses but can also be good for mob control (it usually does 3x to 4x damage compared to drillshot)) to kill and kite bosses. Happy Playing!