Galdera appears to have the durability/survivability advantage. Their destructive capabilities are more or less similar. Thanos might also need to get past his first form and Omniscient Eye before he can reach the final form. Thanos's time manipulation might be helpful, but not sure if he leads with that. Galdera also has his corruption and power nullification that he uses regularly.
Galdera hides his true self behind the Veil of Darkness and the Omniscient Eye and has his other parts fight. But they all basically spam a bunch of status afflicting spells (Sleep, Confusion, Curse, Poison, Death ect), power nullification, statistics debuff. He also nukes the battlefield with a bunch of physical and elemental attacks. Also stated to be capable of causing a massive global flood upon awakening.
Long video for a long battle, but in short, Phase 1 is the Omniscient Eye and the possessed Souls fighting; and the second phase is Galdera unleashing his true self alongside his three ultimate weapons. I'll need an example of transmuting non-corperals. Omniscient eye also has Petrification. If Thanos warped Omniscient Eye out of existence, he would still need to deal with the real Galdera afterwards.