In case you were wondering which minifigs you use:
You can literally do this the moment you load up in Central Park, you don't even have to play the level to select the characters, they are already in the Character Select menu that you can access by holding
.
After selecting one character to be Doctor Voodoo, tap and then hold
to switch your buddy to Loki. Hold
while aiming at Doctor Voodoo and Loki will freeze him in a block of ice.