Even though this guy is harder than every other boss on the map even though he is a miniboss, you can cheese him really easily! You only need a velonsaur and grappling hooks. Before you summon him, use the grappling hooks get to a high ledge in the arena and put your velo there, once you summon him, go grapple back to your velo and start shooting him. he cannot reach you. Back up into a safe place when he begins to pile all of his stuff into one ball, that is his ranged attack. after he throws it at you, go back and repeat. He drops 30 element and some nice loot.
It is the Hardest Mini Boss because of his Armor, but I used a High level Velonasaur with melee point and I took him down, an army of magmasaurs or rock golems could probably do it to, maybe a Yutyrannus can improve your chances. Up if you found helpful.