Here is the full list of all 44 Flat Heroes achievements.
Perform a Dash - Levitation - Dash Attack
Unlock All Palettes
Perform 3 Triple Kills on battle mode in the same play session
Survive longer than 90 seconds on arrows survival
Survive 60 seconds on arrows survival without attacking
Unlock All Modes
Survive longer than 3:30 minutes on phases survival
Kill 30 arrows with a single attack
Complete all levels
Defeat the Hero World 10 Boss
Defeat the Hero World 9 Boss
Defeat the Hero World 8 Boss
Defeat the Hero World 7 Boss
Use the Super Bomb against the hero arrows on Waves
Defeat the Hero World 6 Boss
Keep at least 1 arrow alive for the first 30 seconds on solo
Survive longer than 2 minutes on steep survival
Defeat the Hero World 5 Boss
Defeat the Hero World 4 Boss
Complete all regular levels
Defeat the World 10 Boss
Defeat the World 9 Boss
Kill 100 bubbles in a combo
Defeat the World 8 Boss
Survive longer than 2 minutes on daily survival
Survive longer than 1:50 minutes on phases survival
Survive longer than 90 seconds on solo survival
Survive longer than 2 minutes on vertical survival
Survive longer than 4 minutes on mix survival
Kill 4 tracers in a single attack
Defeat the Hero World 3 Boss
Defeat the Hero World 2 Boss
Defeat the World 7 Boss
Defeat the World 6 Boss
Perform a "Dash Attack" five times in the same play session
Defeat the World 5 Boss
Kill a tracer while on air
Defeat the Hero World 1 Boss
Don't touch any surface for 10 seconds
Defeat the World 4 Boss
Defeat the World 3 Boss
Perform a "Triple Dash" five times in the same play session
Defeat the World 2 Boss
Defeat the World 1 Boss