Played on PC.
Boss: Cala Maria
Loadout: n/a (plane level)
Charm: Whetstone
The Cuphead Souped Up mod is an incredibly difficult Cuphead mod that gives bosses some new attack patterns, makes them faster, and gives them way more health. There's also a stat rebalance of the weapons in the game to nerf some of the most powerful weapons and buff some of the under utilized weapons to allow more versatility in weapon choice for each boss.
This is probably the single best level for Whetstone in this mod. Cala Maria goes from being an upper tier boss in terms of difficulty to pretty mid tier with the Whetstone charm. You can parry the pufferfish in first phase so you don't have to waste time dodging them, plus all the added damage from Whetstone really reduces the time spent in each phase. Getting up close and personal to use Whetstone in the final phase is high risk, but the final phase ends up going down in a matter of seconds with all the constant damage.
The mod can be downloaded here: bit.ly/cup_mod -- Watch live at / jason2890