To unlock the Umbrella trophy in Assassin's Creed Liberation Remastered, you will need to kill 25 enemies with the Parasol Gun.
You won't have access to the Parasol Gun until later in the game when you are given the weapon for the Lady Persona by Blanc, your associate. By default, you will only be able to carry five poison darts (ammo for the Parasol Gun) at once. You can increase this to 20 by upgrading to the Biggest Bag.
Once you have it, simply kill 25 enemies with the Parasol Gun and you'll unlock the trophy.