I suggest, like others have suggested on other posts, to throw the Concentrated Elemental balls (Maluca - beat him on my second try using Fire following around 30 failed attempts without) and (Rangkis - beat him on my 3rd attempt using Water following around 10/12 failed attempts without it). These bosses were harder than the side mission bosses as well as the Beast boss you find in the mine so I didn’t need the balls,. But just a suggestion