Here is the full list of all 20 Plexus achievements.
Find all weapons
Make a Monster pulled into the black hole
Kill a boss
Die 100 times
Kill 6 monsters in a second
Kill a monster
Find all relics
Destroy 2 bonuses on a level
Kill 2 monsters with your corpse
Kill 100 enemies
Kill 1000 enemies
Kill 9000 enemies
Survive with 1 HP
Complete a biome
Charge Athena with a lot of mana
Kill a monster with a ram
Die 10 times trying to complete level
Complete 20 levels
Complete half of levels in every biome
Complete the game