I couldn't finish 'em. Looks like this's gonna get complicated.
Galvanth will primarily attack by spamming Water, as well as occasionally using an instant cast Drain Touch as a damage spike and life drain. Luckily, the healing provided by Drain Touch is inconsequential compared to his much larger health pool.
At 80%, 60%, and 30% health, Galvanth will summon an Inconspicuous Imp and other monsters. As long as the imps are alive, Galvanth will become immune to all damage, making killing the imps a top priority. On the first occasion, Galvanth will only summon one imp which can be quickly taken down. On the second occasion, he will summon two Skeleton Soldiers and a Deepcroft Miteling in addition to the imp, and on the third occasion he will summon two skeletons, two mitelings, and most importantly two imps, which must both be killed in order to attack Galvanth again. Killing the skeletons and mitelings is not necessary to bring down his invulnerability, but they have very low HP and can be easily AoE'd down.
Galvanth's most dangerous ability is Mind Blast, a large circular AoE around himself that deals moderate damage and paralyzes anyone it hits. He will first use it at 50% HP and then at irregular intervals afterwards. Mind Blast cannot be interrupted, and as such must be dodged.