Here is the full list of all 20 Rain of Arrows achievements.
Killed the Drowned Knight.
Killed the Kraken.
Killed the Titan.
Killed the Reaper.
Killed the Trickster God.
Killed the River's Fury.
Killed the River's Fury on the highest difficulty.
Killed 666 enemies with explosions.
Landed 100 headshots.
Beat a level without taking any damage.
Destroyed 100 explosive barrels.
Blocked 1000 shots.
Shot 50 birds.
Collected 100 treasures.
Destroyed 50 bridges.
Killed 300 enemies with the piercing shot.
Killed 1999 enemies.
Healed yourself for 500 points of damage.
Dodged 100 shots while time was slowed down.
Shot 100 enemy fireballs.