Phantasm Dragon

You never find one wandering. The Lunatic Cultist calls it in during his ritual attack, and he does it as a punishment: strike one of the fake copies instead of the real body, or take too long picking the right one, and the dragon arrives. Make the same mistake again while it is still alive and he sends an Ancient Vision instead, so a sloppy fight leaves you juggling the boss and two summons at once.

It flies like a Wyvern: long body, no regard for terrain, constant passes across your arena. The whole thing is one chain of segments, so anything that pierces or explodes is worth several times what a single target weapon gets. The Solar Eruption is the obvious pick if you cleared the Solar Pillar on an earlier run, and whips, the Electrosphere Launcher or a Toxic Flask all land multiple hits per use. Its bulk suits life steal too, since the Vampire Knives connect across a lot of body at once. Aim for the head to kill it fast: softest part, and the worst part to stand near.