Sully made armed with guns police officers scream so loudly and run with such terror that it destroyed the entire scaring lab in Monsters University. Grown men and women with guns and shit.
in Monsters U as a teen he managed to generate enough scare energy from ADULTS that it opened a closed trans-dimensional from the human side and blew out an entire room of those scare battery things
An impressive feat, but even James admitted that he never could have pulled it off without the expertise of Mike Wazowski and his use of scare ambience to set up Sullivan's attempt. That was such a well done scene, too.
In hand to hand, I imagine Sully's, at base, within Shrek's ballpark of strength, but without the feats to back it up, round one goes to the onion.
Round two is squarely in Sully's corner though. Both pack a good pair of lungs, but Shrek only really uses it to keep himself in comfortable solitude, and between Monsters University and Monsters Inc., we see that Sully and other Pixar Monsters are literally raised for scaring. It's not just something to do, it's what keeps their entire society functioning comfortably.
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