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Stay on the ground the whole fight. Use Pierce and the stone pillars by the entrance for cover until you get Tomassi down to almost halfway and other Hiss start to appear. At that point, run to the back right corner (from the entrance) and hide under the overhang. It gives you almost complete cover and there's a stone cover out front of it too. Let them come to you. Switch to Shatter and take them out as they get close. Jump out and grab health when you need it. Look for the signs of the invisible hiss and throw stuff at it to make it appear. Focus on killing the invisible ones first or they will trap you in the corner. Also watch out for flying hiss trying to explode. You can run back into a corner to avoid the explosion if you need to. When you have openings, get out behind the stone and shoot Tomassi with Shatter. Keep whittling down his health until you get him.