Don't listen to the Dododex listing, Magmasaurs are the BEST metal gatherers and it is worth it to steal one. A 100 melee gets way more metal than my 500 melee Ank did. Just grab a few Pteras you don't care about and head in there. The level doesn't really matter because any Magmasaur will gather more metal than an Ank.
Also, when you get a Magma, you don't have to worry about eruption because it is immune to it.
Magmasaur is able to destroy ALL STRUCTURES utilizing its searing spit. While it is a walking forge it produces metal at the same rate as a refining forge. You can use the jump button to taunt other creatures and when holding searing spit use crouch to erupt. (This attack pushes creatures back and leaves magma everywhere) it is breedable in lava. Hope this helped