Bone Pen wasn't hard for me, I beat it on my first try randomly clicking Pull Stars. Luigi's Purple Coins , and definitely the Bouldergeist run, were genuinely hard, but my votes for hardest are the Dreadnought trash dump and Rolling Gizmo Galaxy objectives.
I'm also with Kimimaru on the Sea Slide one. I say, SMG2 should be time-consuming because of difficulty, not boring, long but not particularly hard challenges. And if Luigi returns, the 121st star objective should be a painfully long and hard level making use of both Bros. and multiple powerups, not another Grand Finale Galaxy-esque objective.