The other two guides did NOT work for my group. Leading that dang fireball into volcano is NOT the way. The boss needs to MASH the MAGMA ball as it is floating towards a player. So a player gets targeted, the fireball is lead in front of the boss as he performs the mantle breaker attack which causes the geyser eruption. I take no credit for the video but it shows the achievement way better
Video by NainoaKumaaHoku: