Atomos (Final Fantasy V)

Atomos is a boss in Final Fantasy V. He is at the top of the Barrier Tower outside Exdeath's Castle, guarding it from intruders. Atomos is famous for being a very tough boss, so much so, that he became a summon in later games.

Strategy
Atomos' only attack is Comet. He will keep using the attack until he KOs one of your team members. However, once he does KO a team member, he will stop using Comet, and instead start sucking the KOed character closer. Once the character is close enough, he'll use Wormhole, removing them from the battle. After he does, he will start using Comet again until he KOs another member.

As such, speed is important here. Since Comet's damage doesn't factor in defense, defense isn't what you should be focusing on. The best strategy is to let him kill one of your characters, then attack him while he's busy sucking them in. Of course, you shouldn't just let him kill somebody. Have a White Mage constantly heal the party, while the other three members attack with the best attacks and spells you have. Once somebody is KOed, have the White Mage fully heal your other members, while they continue to attack. If your White Mage was KOed, don't bother healing, and instead, just focus on attacking. You should be able to defeat him before he sucks up your first character. If you can't, don't worry. Just keep on attacking him, and if he didn't suck up your White Mage, you should be able to defeat him with no problems.

Alternatively, Atomos isn't immune to sleep, so putting him to sleep and attacking him with spells can be a solution. For a even cheaper strategy, cast Black Shock on him once, then L5 Doom.