Here is the full list of all 18 DEXED achievements.
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Complete underwater with a score above 600,000
Complete forest with a score above 800,000
Complete volcano with a score above 900,000
Complete snow with a score above 1,000,000
Complete boss with a score above 500,000
Kill the boss on hard without missing any targets
Tag 20 enemies in a single combo
Reach X20 multiplier
Kill all enemies in the underwater level in Easy or Normal
Kill all enemies in the forest level in Easy or Normal
Kill all enemies in the volcano level in Easy or Normal
Kill all enemies in the snow level in Easy or Normal
Destroy all the targets on the whale
Kill all enemies in the underwater level in Hard mode
Kill all enemies in the forest level in Hard mode
Kill all enemies in the volcano level in Hard mode
Kill all enemies in the snow level in Hard mode