You must pay 10,000 Florins to receive the training for Long weapons at your training grounds at your Villa in Monterigionni. After you learn the sweep technique, I recommend you try to perform this in either Tuscany or Florence where there aren't too many advanced enemies. Once you're in one of these towns annoy the guards so they begin too pursue you and then roam around, not too fast, so you don't lose your pursuers until you find a spear guard. The more guards you have the better your chances. Make sure you are unarmed and wait for the spear guard to attack and press square to disarm him. This will only work if your are unarmed otherwise you will counter. Once you have his spear, dont run away if you sprint you will drop the spear. Instead use your strafe button X to move a safe distance away and charge your attack by holding the square button. Once they are close enough let go of square to perform your attack. It will take some tries but once you hit five the trophy will sound

Here is a video of someone getting the trophy
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