After Building All of The Buildables , You Need to Take All of Them And Use All of Them İn The Same Time . Go Take Sentry Turret And Use İt , Then Go To Boom Box And Use İt . Do This To All of Them İn The Same Time . You Will Be Fine Also You Can Watch Some of The Guides For This Trophy İn Youtube
He means use them in the same GAME, as you obviously can't use them all at exactly the same time. Just build them all, pick them up, and use them. Toy Robot, Boombox, Lava Lamp, Sentry Gun and Zom-B-Gone.