Bell Reve

Use a heat beam to melt the gold cell door, shoot a mirror and then a minion will jump through the hole into another cell. Drop down to the previous level and shoot the lower mirror even though it should still have a portal. There's also another cell with a mirror on the back wall, so shoot it through the hole in the cell door and the minion will escape. Switch to Joker and recruit the minions pulling down a cat toy. Build the glowing pile of bricks to create a safe bridge to the right. Keep going until you're against a wall where you'll find a gray panel that can be pulled with all three grappling guns. With the panel out of the way you'll have access to a drone door so summon Joker's drone and drive it in, then explode it against an silver machine in overwatch room as this will summon an elevator which takes you up an overwatch room so collect Gold Brick