Here is the full list of all 24 Agartha achievements.
Complete every achievement
Clear the last stage
Unlock all stages
Unlock all characters
Collect 30 jewels
Collect all jewels
Collect 100 meat
Collect 200 meat
Clear a stage by passing through a hidden goal
Clear the Shark boss stage (D-1)
Clear the Ice & Fire boss stage (B-5)
Clear the Golem boss stage (D-6)
Clear the Dragon boss stage (E-7)
Clear the Haniwa boss stage (G-6)
Clear stage H-1
Clear stage A-7
Clear stage E-8 as Gunner
Clear stage H-5 as Wizard
Clear stage A-8 as Esper
Defeat all the enemies and clear F-1 as Ninja
Clear stage G-1 only using Cultivator as Farmer
Clear stage F-3 as Alchemist
Clear stage E-3 as Geomancer
Clear a stage using Self-Destruct as Robot