Here is the full list of all 27 Rusty Hearts achievements.
Complete the "Study Gloves" Quest
Complete Gorgon's Quests
Complete Joheim's Quests
Complete the "Not My Box" Quest for Gorgon
Complete the "Not My Box" Quest for Gina
Complete Estel's Quests
Complete the "Not My Box" Quest for Nadia
Complete the "Return to Nadia" Quest
Reach Level 10 with any Character
Defeat the Lobster King Boss in the Lower Plant
Complete the "Scout the Lower Plant" quest
Complete the "Ryan's Request" Quest
Reach Level 5 with any Character
Reach Level 20 with any Character
Achieve 5th Cadet Ranking in PVP
Achieve 10th Ace Ranking in PVP
Defeat Fluffy in the Labyrinth
Complete the "Wanted: Boozatron B" Random Quest
Complete the "Wanted: Dewey Decimator B" Random Quest
Unlock the Cathedal area
Complete the quest "A Simple Task" from Technical Officer Ryan.
Complete Vincent's first quest
Complete the "Scout Canals B2" Quest from Elma
Join the Militia
Slay 100 Skeleton Soldiers in Georg's quest
Reach level 30 with any character
Reach level 35 with any character