The Word Dumpster

This covers hard modes in the Rift chronicles. Bolded red mechanics can potentially one-shot you.

Infernal Dawn: Laethys

Use 1 ST healer and 1 DPS for all fights, or a Defiler if you've really got the fights down. Bolded red mechanics are ones that can potentially one-shot you.

Maklamos

Focus on not getting hit by bombs, burn the juggernauts down ASAP and interrupt their heal, intercept Doom.

Crystalline Missile: Spams aggro target with earth damage, heal through it.
Quartz Blast: Deals ~8000 damage to everyone in LOS. You can hide from it using either of the high edges next to the boss's pillar.
Crystalline Bomb: Emotes what color bomb he's throwing at which players and after a brief cast, launches projectiles at all players that do ~14000 damage unless they're on a gem pile of the same color. When he emotes, you should mount up and run to the gem pile because the cast is very short. Whether you take damage is determined when the missile hits, not when the cast completes. If you fail to dodge, then he will cast Bomb Empowerment and his bombs will do double damage for the remainder of the fight.
Summon Juggernaut: Every 25% HP, he'll shield himself and start summoning a juggernaut. The juggernaut will cast Returning Power, which makes it heal itself. Interrupt Returning Power if you can (preferably late in its cast time) and burn it. If the boss finishes summoning, the juggernaut will begin spamming Fractured Defense, which is a cleansable stacking AOE debuff that increases damage taken. It will also interrupt your mounting attempts.
Doom: Summons a light pillar that slowly moves toward the doomed player. The other player needs intercept the light pillar, which will deal ~1600 damage to them. If this is not intercepted, the target dies instantly.

The Dreadfortune and Rusila

Kill all of the Dreaded Longshots before doing anything else. You mainly want to do 2 things with the barrels: Drop them on hatches to stop adds from spawning, and drop them next to the Heart of the Dreadfortune to drop its shield.

Throughout the fight, saw blades will spawn on the deck which do low damage if you run into them. Purple AOE circles will spawn on the deck that will deal damage and launch you if you're in them when they expire. Once the Heart of the Dreadfortune is killed, Rusila will join the fight:

Repeating Shot: Channelled AOE damage. Can be LOSed or interrupted.
Thousand Cuts: Throws saw blades in 8 directions. Run away from the boss.
Rusila's Fist: Purple ground AOE, will knock you somewhere if you stand in it when it expires. May throw you off the deck.
Inner Wrath + Outer Wrath: Rusila will teleport to her off-aggro target and cast Inner Wrath, which knocks back and does damage to anyone inside the AOE, then casts Outer Wrath, which does the same to anyone outside of the white ring. Getting hit by Inner Wrath usually means getting hit by Ouer Wrath and can easily throw you off of the deck.
Lava: Don't stand in the fire.

Laethys

Phase 1

Flare: Interruptable projectile nuke, does ~10000 damage to a random target. You can interrupt with the reactive.
Molten Gold: Creates expanding AOEs that do ~7000 damage per tick. Avoid.
Golden Breath: Frontal cone AOE on aggro target, sidestep it.
Greed's Wrath: DOT inflicted on anyone that does damage to Laethys. Heal through it.

Phase 2

Gold Piles + Laethic Gold + Golden Destruction: Stand on the gold piles before she finishes casting Laethic Gold. She'll then cast Golden Destruction, which deals AOE damage to anyone in LOS in the room. Hide behind gold piles to avoid.
Rear Guard: Summons powerful adds if you get behind Laethys.
Silver adds: Summons some silver adds over the course of the fight. The adds do increasing damage the longer they are alive and should be killed if the boss is at high health.
Tail Lance: Purple pulsing AOE that deals damage and knocks back after a delay.
Auric Evocation: Deals insignificant damage in a sweeping area in front of the boss.
Slam: Boss will cast a slam either left, right or center. These directions are relative to the boss, so "left slam" will hit to the right side of your screen if you're facing Laethys. Does ~50k damage. Completely avoidable by DPSing at range.

Planebreaker Bastion: Aftermath

PBB is more varied and has more one-shot mechanics.

Usurper Esai

Above all else, do not get hit by the one-shot mechanics. Having a DPS with a lot of instants is recommended as the fight is very movement-heavy.

Burning Ground: Cast at the start of combat and lasts until the boss is dead, deals ~3400 damage to everyone not moving regularly.
Grit Blast: Cone channel centered on aggro target, deals significant damage. Can be avoided by standing in an Entropic Zone (one of the golden energy fountains on the side), non-targets can stay out of the cone. As soon as he's finished casting this, he will cast...
Frenetic Space: Fire tornado centered on aggro target. The aggro target should be moving when he casts this because it doesn't last for long. Anyone inside the tornado when it detonates will be thrown off the platform and die instantly.
Kinetic Redirect: Roughly the time this finishes casting, possibly slightly before or after, anyone that isn't standing still will be thrown off the platform and die instantly.

Sharad X9

Dance floor fight, bring instants again. For how complicated this looks, it's not very hard.

Standard Slug: Deals ~6000 damage to off-aggro target. Can be LOSed using the pillars. Interruptible.
Heavy Slug: Deals ~10000 damage to off-aggro target.
Laser Net: Rotating laser beam disco.
Blast Ring: Deals very high damage to anyone too far from the boss.

Tesni the Ravenous

DPS should use a tank pet! Using a tank pet will completely prevent the Spew mechanic.

If you're not using a tank pet, then it's very difficult to avoid getting knocked off of the edge by Spew while trying to pop a bug mound. Someone should bring some AOE to deal with the small adds.

Spew: Does low-moderate damage and knocks back by about half of the platform width. If you are pushed off the platform, you die instantly, avoid standing near the edges if you don't think the boss can be pointed away from the center (i.e. after Spitting Mad).
Spitting Mad: Creates a large number of ground AOEs on players. Each one will do ~7000 damage per tick.
Acid Splash: Both the healer and DPS need to be near a bug mound and the DPS needs to destroy it. If you're near the bug mound, you get a buff that will protect you from Acid Splash, otherwise you take 3 ticks of ~10000 damage.
Big add: DPS down when convenient
Lots of small adds: AOE them down, they're very weak but do significant damage.

Mistress Inyra

Pretty much all of her mechanics are one-shots except for her Planar Bolt plink damage ability, so your healer can go defiler for this fight. This is mainly a DPS race.

Planar Bolt: Plink damage on aggro target.
Call of the Planes: Summons 6 Empowering Blobs. These increase her damage if they reach her, but there's very little you can do about it. Once all blobs reach her, she will cast...
Death Blast + Death Ring: Ring areas with a ~10m radius. Either the inner or outer area will be red and pulsing, you want to stand in the area NOT pulsing or you will take significant damage, up to 45000 later in the fight. She may randomly cast one of these throughout the fight as well.
Earth Blast: At random times, will start alternating quadrants of an X-like pattern, dealing damage in the red area. She can also change her facing direction during this, so your safest bet is to be close to her.