I've returned on playing SMT IV after a while and I've been trying to beat the all the DLC Bosses (the four Archangels, Ancient of Days, Sanat, Masakados). I've defeated Sanat just now and he was a pain in the ass, it took me three tries to beat him. I really hate Gaea Rage and he loves spamming it, for those who don't know it deals heavy Almighty damage (an element that no one can null or resist) but also can get demons Lost (they get blown away from the battlefield and you have to go through an encounter to get them back). Sanat also nulls everything but Gun and Almighty.
Since I was always the last one left in the fight, I decided to grind to level 99 (Sanat was 95) and upgrade Megidolaon to +7 (could have maxed it to +8 but I was already broke). My party was Ancient of Days, Vishnu, and Shiva. Because they either get Lost or die in the first few turns, their only purpose is to buff me and debuff Sanat. I didn't bother charging up Megidolaon since it was already dealing a good amount of damage . After Vishnu and Shiva got Lost and Ancient of Days got killed, I took on Sanat solo.
The entire battle at that point was me and Sanat throwing attacks at each other, he spammed Gaea Rage for all his turns (he gets to act three times in one turn) and I was spamming Megidolaon at him. When I was low at health or MP, I would heal up and continue attacking him. Rinse and repeat. Eventually, I managed to finally beat him and it felt great. Here's a pic of his defeat:
This fight had worn me out so I stopped playing after that. I'm going to take on Masakados tomorrow so hopefully I can beat him too.