Magma drops 6 Rare/ExFrag Rocks. Each person completing the mission will need one so one Adaman Ore would be enough for 6 people; if more than 6 people need the mission, it will need to be killed more than once. Adaman Ore can be bought from Auction House or acquired via several other methods.
Magma needs to be killed to get the Frag Rock, but the Salamander does not. To fight only against Magma, the Adaman Ore should be traded while having the effect of Sneak on. This way, only Magma will aggro. NMs seem immune or at least highly resistant to Sleep.
As soon as the Magma attacks you after spawning, your Sneak will wear and the Salamander will aggro you, making this a very unsafe method if you're soloing.