Queen Slime

She takes a Gelatin Crystal, used in the Hallow or the underground Hallow, at any time of day. That is the whole requirement. She is optional, nothing is gated behind her, and she can be killed in early Hardmode before you touch an altar or a mechanical boss.

The biome only matters for the summon itself. Once she is out you can drag the fight anywhere and she will not enrage, so the usual play is to pop the crystal at the edge of the Hallow and retreat to whatever arena you already built. Do it in daylight if you can, because her projectiles are pink on black and close to invisible at night.

She hops toward you in fast bursts, much quicker than the king manages, and if terrain gets in the way she sinks into the floor and reappears closer, or directly overhead if there is nothing solid nearby to land on. Recent versions let her teleport onto platforms as well, so a raised arena no longer locks her out of reaching you.

After a run of jumps she stops and picks one of two attacks. Either she charges the crystal on her back and throws out a spread of gel blobs that fall under gravity and bounce off surfaces a couple of times, or she phases up through the ceiling, lines herself up over your head and slams down, spreading a shockwave along whatever solid ground she lands on. Both are slow enough to read. Liquids drag on her and on the blobs, which is worth knowing if there is water in your arena.

At half health she grows wings and stops touching the ground. Teleporting stops with it, she flies through blocks instead, and both attacks change shape. The gel spread becomes a full ring that forms around her and then opens outward, so sitting at one fixed angle no longer avoids it. The slam becomes a dive she aborts and retries if you are not standing over solid ground, which is exploitable if your arena has gaps in the floor. Liquids also stop slowing her down, except while she is committing to a stomp.

Every hit she takes spits out one of three slimes, so your own damage output controls how crowded the screen gets. The crystal ones are the dangerous type: their spikes travel in tall vertical arcs, which means the airspace directly above a cluster of them is where players die. The bouncy ones lob gel along the ground and are mostly noise. The flying ones only want to touch you.

Clearing them takes priority over damaging her. Anything piercing does the job in a single shot, and stacking two or three damaging debuffs will finish any of them from one hit that lands. Royal Gel does not make them ignore you, which catches out almost everyone the first time. None of them fire while floating on liquid either, so a shallow pool is a free crowd control tool.

Use short disconnected platforms rather than one long floor. Ground minions fall through the gaps and out of the fight instead of stacking up under you. A lava floor works even better, since she is immune to it and they are not, provided you have Lava Waders or Terraspark Boots so you can stand in it yourself.

Height management is the other half of it. Stay airborne through the ground phase, because most of what she throws down there cannot reach altitude. When the wings come out, drop back toward the floor and pull her up now and then, since her flight is slow and predictable and her projectiles are much easier to walk out of at ground level.

Piercing darts clear minions and hit her several times on the way through, which covers both problems with one weapon. Life Drain is the standout for magic users: it damages continuously around the cursor, catches minions the instant they emerge from her center, and feeds regeneration back to you while it is connected. The Daedalus Stormbow with Holy Arrows blankets the whole arena. An Invisibility Potion stops the flying slimes tracking you and lowers background spawns, but it drops the moment anything touches you.

In a for the worthy world her gel volleys get noticeably larger in both phases and the direction of the spread is randomized instead of fixed, so pre-aiming your dodge stops working. Her stomp also travels much faster, which turns the one attack you could reliably walk out of into a real threat. In Master Mode the Black Spot mount lets you hover above everything she does in the first phase.

The armor she drops is the reason to fight her early. It favors no particular class and gives a survivability bump exactly when early Hardmode is at its most hostile. Most of her table is an upgraded version of something the king leaves behind: a mount, a hook, a novelty weapon and a set of armor. She can be resummoned as often as you can craft crystals. If King Slime is also dead, the Princess adds a new item to her stock. She does not count toward the achievement for beating every boss, and her treasure bag never contains developer gear.

One despawn quirk worth knowing: she has to physically leave the screen to give up after killing everyone. If blocks pen her in during the first phase she cannot manage it, and the fight simply continues when you respawn.