These can drop on Normal Adventure mode. Got one in Corsus, Needle Lair only a few minutes into the dungeon.

I only saw one comment about it after scrolling for a while here so I wanna point something out. I was just farming these and practicing my skills by rerolling an earth adventure on the second hardest difficulty. My main goal was really to get better at the game but I wanted simulacrums as well. I just killed ent not 5 minutes ago and most certainly got a simulacrum as a drop from him. almost certain it's not a but but almost certain it's stupid rare as well. Not an effective farming method but it's definitely something that should be written here

On Earth, another of the named locations for the Simulacrum is Research Station Alpha (Leto's Lab). Preceding the Riphide boss.

I will give the best descriptions that I can as to where they are located, but it requires some forehand knowledge of how the worlds are arranged. I will provide short "maps" for each world.

EARTH:

2 main areas: Fairview (1st) and Westcourt (2nd). 1st area contains 1 dead-end dungeon, 1 boss dungeon, and the Subway connecting to the Church, then to the 2nd area. 2nd area contains 1 dead-end dungeon, 1 boss dungeon, and the World Boss connecting to the Keeper's Tower. The boss dungeons allow access to the other side of the middle divided area.

The Earth Simulacrum is located in the Westcourt (2nd) Pre-boss dungeon.

Names included: Sunken Passage, The Hidden Sanctum, Cutthroat Channel, The Tangled Pass.

ROHM:

2 main areas: The Eastern Wind (1st) and The Scouring Wastes (2nd). 1st area contains 1 dead-end dungeon, the entrance to the Undying Throne, and 1 boss dungeon that connects to the 2nd area. 2nd area contains 1 dead-end dungeon, 1 boss dungeon, and the World Boss dungeon that connects to the 1st area.

The Rohm Simulacrum is located in the Pre-World Boss dungeon.

Names included: Loom of the Black Sun, unnamed/blank dungeon that leads to Harrow.

CORSUS:

2 main areas: The Fetid Glade (1st) and The Mist Fen (2nd). 1st area contains portals from the Undying Throne and the Labyrinth, and an entrance to The Shack. 2nd area contains The Elf Queen, 1 boss dungeon connecting to other side of the Queen's gate, and The Grotto / Ixillis.

The Corsus Simulacrum is located in The Fetid Glade. May take longer to find due to size of the area.

YAESHA:

2 main areas: The Verdant Strand (1st) and The Scalding Glade (2nd). 1st area contains 1 dead-end dungeon and the entrance to the Shrine of Immortals which connects to the 2nd area. 2nd area contains 2 boss dungeons, 1 dead-end dungeon, and the World Boss dungeon.

The Yaesha Simulacrum is located in the Scalding Glade. May take longer to find due to size of the area. Note that this area is split by a divider and you may need to clear a connecting boss dungeon to gain access to the side containing the Simulacrum.

Credit:

I’m on my first playthrough and just beat Singe. I guess I keep getting lucky because I now have 5 of these

The absolute fastest way to get simulacrums is corsus adventures with the rusted amulet method. Having a 3 man party would make this insanely fast because of having 3 keys. The amulet once looted, every partymember gets it and you can recieve 3 keys each. rerolling for fetid pool every run will probably net you about 20 - 30 simulacrums an hour depending on how efficient you run the adventure. P.S. Any difficulty is fine.