Although this can be earned at any time, you can easily unlock this trophy during the pirate fights of Chapter 1. For the first two enemy boats, simply shoot the explosive barrels to destroy them easily. Starting with the third boat, Pirates will start jumping into the water and boarding your boat. As the tutorial will show you, you can unleash a Brutal Combo
by getting within melee range, and do a three attack combo with
. Your attack will have a special animation if you do it correctly. Continue using Brutal Combos on every pirate until you've killed 5, and the trophy will pop.
As far as time is concerned, the kills don't need to be immediately one after another (although you should keep them as close as possible), and you can even mess up a punch or two, so long as you deliver the killing blows with a Brutal Combo.
Thank you for positive votes! Please comment if I've made a mistake or you have anything to add. Stay tuned, as I'll be adding many more for Uncharted: The Nathan Drake Collection as I play and unlock them. Feel free to add me on PSN! Happy Hunting :)