Armored Core VI: Embers of Rubicon Final Boss Strategy Guide: Drich, Coral Matriarch in IBIS REEF.
The ALLMIND route final boss is widely considered the hardest fight in the game by a huge margin. How to beat her?
Short version: Get Good.
Long version: This is the most diabolical boss FromSoft has ever made. Possibly the most diabolical boss in modern gaming. IBIS REEF alters its movepool and statline to focus on whatever bodies you the most. The game has been recording your deaths across all routes and using them to select what weapons IBIS REEF is carrying, what extra customization it has and what the drones will do. You basically have to shore up your weaknesses or get lucky. Or play a lot of multiplayer to get practice without IBIS REEF adapting. And yes, IBIS REEF's move selection adjusts based on your defeats against IBIS REEF.
Things to watch out for, shared across all versions:
Shields: IBIS REEF always has a shield like BALTEUS's ... except IBIS REEF doesn't get staggered when its shield goes down. Get used to the shield coming back up at very inconvenient times, if IBIS REEF detects your loadout has weapons with excellent punish capabilities but limited effect when non-staggered and excellent stagger but limited direct damage it will delay shield reactivation to lure you into exploiting a window that won't happen sometimes.
Counterplay: ...It's like any other shielded machine, only breaching the shield doesn't create a punish window. Avoid the punishment for getting greedy by not overcommitting.
Boost-kicking: It's a pseudo-AC, Drich will boost-kick you and is very fond of countering your boost-kicks with hers. If you die to boost-kicks a lot, she'll do it more and may add foot mounted coral blades for extra reach.
Counterplay: Basically, get good. If this kills you a lot, Drich will add the coral blades and do it more, and at that point you just have to learn to dodge it. Punishing the charge only works if the shield is down or damaged, and the bladed kick out-reaches basically everything melee so forget hitting first.
Drones: What IBIS REEF's drones do depends on what's been bodying you, but IBIS REEF will always have attacks coming from well outside of its hitbox and Drich will always at least try out having the drones act like Orbit and Drone type weapons because you won't have shown that won't work - there aren't enough Drone/Orbit using enemies for that. The Drone mode is probably IBIS REEF's least damaging attack, but it's also one of the most dangerous. It's hard to dodge 16 guns floating around you shooting from all angles and stagger doesn't stop it. The only way to not have to deal with this is to get bodied by IBIS CEL's drone laser barrage so much that REEF focuses on equivalents of that attack.
Counterplay: Quickboost spam is good for getting out of drone storms. Orbits are just more firepower from the same vector, but beware that you don't lose your punish windows to them firing while IBIS REEF is staggered. Assault Armor bursts can disable the drones for a time but this is not worth the use alone.
Terminal Armor and Repair Charges: Unlike every other pseudo-AC boss, IBIS REEF has repair charges. And Drich will use Steel Haze Ortus's trick ... and both resources are on a cooldown. You have thirty seconds to finish IBIS REEF after Terminal Armor - Repair Charge activates before it can do it again.
Counterplay: Finish the fight. 6k AP in 30 seconds is not that hard if you were able to bring IBIS REEF down to zero in the first place.
Coral Weaponry: Everything IBIS REEF does except for unbladed boost kicks does Coral damage.
Counterplay: Damage type based defense ratings don't matter, ignore them when building your loadout.
Psychic Damage: Alien Mom's battle dialogue hurts because she's calling you out, hard, after you're probably familiar with and like her. It's like Ayre or Ezra on Overseer, only worse.
Counterplay: Harden your heart, or take her last minute offer of an out and go to NG without a proper ending.