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