Ichor Bullet

The defense strip is the whole point. Against anything with real armor that reduction is worth more than the bullet itself will ever be. The way to use it is to open with a short burst, swap to something heavier for the body of the fight, and flick back when the debuff runs out. That beats firing these exclusively by a wide margin, since Crystal Bullets and the like hit far harder once the target's defense is already gone.

The Destroyer, the Lunatic Cultist and every Celestial Pillar shrug the effect off completely, which takes this out of most of the endgame. Bring something else to those fights. Shots reflected back at you also lose the effect, so an enemy that returns fire will not stick you with it.

Worlds with Corruption get Cursed Bullets instead. Those burn rather than strip armor, so the tactical pattern above does not carry over.