Flask of Nanites

Confused enemies walk away from you instead of toward you. In a corridor that turns a wall of bodies pressing into your face into a scattered mess drifting the wrong way, which is space that melee normally cannot make for itself. It does not reduce damage or stop anything dangerous outright, so treat it as crowd control rather than defense.

The post-Plantera dungeon and the temple, both of which are tight rooms where being surrounded is the actual threat. Against a single boss it does almost nothing, and most things worth calling a boss shrug the debuff off anyway. Since only one flask can be active at a time, running this means giving up the raw damage a Flask of Ichor would have added, so swap it in for the rooms and swap back for the fight at the end.

Melee weapons that throw a projectile still spread it on the projectile hit, and whips carry it as well. That last part matters more than it sounds: a summoner tagging targets with a whip is applying confusion across the whole group while the minions do the damage.