Thanks you Pichu, I'm looking for a quick move-set I made on the bowser junior thread for shadow mario entirely unique from mario. Turns out it was just a final smash:
For final smashes, I was thinking for shadow mario, he could call upon the phantamanta (fought on the first level of sirena beach in super mario sunshine) to uh... swim? around the stage for 20 seconds, and shadow mario could attack it and make it split in half etc. like in super mario sunshine, but the phantamanta still gets smaller when you attack it. The size of the phantamanta depends on the size of the stage you are on, so it would be much smaller on big blue than on new pork city, where it would have to be giant.