Queen Slime

Daedalus Stormbow + Holy Arrows is the top pick for Ranger — the arrows and falling stars hit Queen Slime and all her minions at once, dealing massive total DPS. For Melee, grab Shadowflame Knife for safe ranged kiting or Death Sickle for piercing damage. Mages should use Meteor Staff for main DPS and Golden Shower for the Ichor defense debuff. Summoners want Sanguine Staff from Blood Moon fishing — the bat AI tracks Queen Slime perfectly through both phases.

Find a Gelatin Crystal in the Underground Hallow. It's a rare pink Crystal Shard variant (about 1 in 50) that grows on Pearlstone or Pink Ice. Mine it, then use it anywhere in the Hallow, surface or underground. There's no time-of-day restriction. Since 1.4.4 it stacks to 9999, so grab spares for refights. You need to be in Hardmode for the Hallow to exist.

Every Classic kill drops one Crystal Assassin piece (33.33% each, full set gives a dash and 36 defense), plus a 25% Gelatinous Pillion, 33.33% Hook of Dissonance and 25% Blade Staff. The Expert/Master Treasure Bag raises the Pillion to 50% and the Blade Staff to 33%. The Crystal Assassin dash frees up your Shield of Cthulhu slot.

Build it inside the Hallow, because the Gelatin Crystal won't work anywhere else. Make 3 layers of platforms, each at least 120 blocks wide with 15-20 block vertical gaps, and put Campfires and Heart Lanterns on every layer. In Phase 1 you stay above her jumps; in Phase 2 she flies through blocks, so the platforms are for strafing, not hiding. Heads-up for 1.4.5: she can teleport onto platforms now. A surface Hallow spot gives you the most open sky.

Yes, Queen Slime is completely optional. She is not required for progression to the Mechanical Bosses or any other boss. That said, her drops are genuinely useful — Crystal Assassin Armor gives a free dash, and Gelatinous Pillion is one of the best early Hardmode mounts. If you can beat her, the rewards are worth it.

Titanium Armor is the best all-round pick: it has the highest defense among early-Hardmode ore sets, and its set bonus spins up to 7 shards around you when you attack (50 damage each), which shreds minions that get close. Pick the class-matching helmet. Adamantite works too. Note that Titanium lost Shadow Dodge back in 1.4.0 — that dodge lives on Hallowed Armor now. Summoners can also use Spider Armor for an extra minion slot.

18,000 HP in Classic, 28,800 in Expert and 36,720 in Master. Contact damage is 60 / 120 / 180 and Queenly Smash hits for 80 / 160 / 240. Her defense is 26 in both phases. It doesn't go up when she grows wings, so keep your DPS going the whole fight.

Crystal Shards keep growing in the Underground Hallow during Hardmode, and each one has a 1 in 50 chance to be a Gelatin Crystal. Place long straight rows of Pearlstone or Pink Ice blocks in the Underground Hallow, then come back and mine whatever grew. A Metal Detector shows them since 1.4.4. Classic kills only give one Crystal Assassin piece each, so you'll want at least 3 refights.

Yes. Blade Staff has been a Queen Slime drop since 1.4.1: 25% in Classic, 33% from the Expert bag. It summons Enchanted Daggers with 25 armor penetration. In 1.4.5 it carries a 33% whip tag damage penalty (25% in 1.4.4), so whips add a bit less to it than to other minions.