To unlock the Rain of Arrows trophy in Assassin's Creed Odyssey, you must first purchase the second episode of Legacy of the First Blade (or own the Season Pass) and download Episode 2.
Once you have reloaded the game with Episode 2 installed, you should immediately be able to see the Rapid Fire ability in the second row of the Hunter Tier of your Abilities tree. You need to have at least Level 1 of this ability unlocked; don't forget you can re-spec your character by holding L3 if you don't have any skill points to spare.
Now you just need to kill 10 enemies with this skill, which you will need to assign to a face button. Hold L2 and press your chosen face button once to activate the skill (the view will zoom slightly), then aim and rapidly hit the same face button to fire a bunch of arrows. Kill 10 enemies with this ability to unlock the trophy. You may want to whittle enemies' health down first before you activate the skill as each arrow only takes off a moderate amount of health.