Speak to Gymer at Thrym's End in Zul'Drak once you have used the Jumbo Seaforium Charge to open the Scourge Enclosure that is holding him prisoner.
1x Scourge Enclosure Blown Up
What I'm about to give you is called seaforium. It is a highly volatile explosive with some unique properties. Due to its malleable nature, you will be able to fill in the hinges and cracks of the cage for maximum damage.
Return to your friend and use the jumbo seaforium charge to set him free.
Let's hope he honors the bargain. He's our only hope.