Operation Babel: New Tokyo Legacy

You can only finish this in the post-game, since you won't be able to unlock all of the dungeons until then. Wanted Variants are the strongest enemies in an area, and they are encountered in random battles. You have a better chance of encountering a Wanted Variant if your Encounter Gauge is high. Defeating an area's Wanted Variant for the first time reduces your Encounter Gauge. Here is the list of all of the Wanted Variants and their locations:

• Elder Sahagin (Gakuto Tunel Closed Road East & West, story related)
• Master PD - Academy Station - Xth Subway Line
• Orge - Abyss Subway Line
• Arachne - Muramasa (I found it on the second floor)
• Tamano-no-Maa - Aoi Shine Heavenly Road North & South
• Psychonation - appears in the Haunted Hospital on floors 2F, 3F, and 4F
• Siren - Green Museum (found it in Zone A)
• Death Scythe - Green Museum (found it in Zone C)
• Kaiser Raptor - Green Museum (found it in Zone F)
• Daughter Guard Amatsu - Muramasa Hall (found it on Fifth Floor, can call in for reinforcements)
• Noble Vamp - GHQ Cemetary (found it in South)
• Algol - Catacombs (found it in Jesus Area and Satan Area)
• Derracotta - Catacombs (found it in West)
• General Borg - Geomatrix (found it on second floor)
• Heavenly Maiden - Shinobazu Prison (found it on B1F)
• Voltrus - Shinobazu Prisoon (found it on B5F)
• Rune - Babyle Sphere
• Shadow Chaser - Embryo
• Perfect Daughter - Muramasa Hall (found it on 9F)
• Dinomelo - Arc I & V
• Grand Buster - Arc VI
• Wyvern Omega - Embryo Arc VII, after opening Lethal Hall. I recommend contracting the Nono and Yuyu Clones from the first Lethal Hall fight.