FUNKO FUSION

Invincible can reached from Umbrella Academy's first level, House Hunters. Complete it first, then make sure you have the turret from Battlestar Galactica and the blacklight from Hot Fuzz.

Unfortunately, you will have to most of the level again (cameo levels kick you out of their home level, so you'd be doing it twice even if you did it on your first run-through). Go through the level normally up until you get the key, making sure to unlock the upper door (the one Cha Cha was near). Go in here, and use the mould machine to make the items. Go back down the hall you just came through and go through a doorway on the left (it'll be next to the key door), solving the crime in here with the blacklight. It rewards you with the keycard to the cameo door, unlock it and then bring the turret through, setting it down at the top of the stairs to shoot at a turret switch. This will activate the cameras at the bottom of the stairs, solve that puzzle and you will finally be able to access the level. Get the crown and get ready to meet (title card).

Also, the level contains major spoilers for the comic and show if you somehow aren't aware of anything about Invincible. Just putting that out there!