Final Fantasy XVs Adamantoise. I spent an hour draining the boss with 5 million plus HP to half health, then, I decided to use the ring of Lucii and ended up insta killing it. It was cheap, but I really didn’t want to fight him for another whole hour.

Beating The End by waiting a few days for him to die of old age

Because I fail at nuzolckes, in a sapphire extreme randomized nuzlocke, I forgot to heal up so my sweeper didn't have any moves left. So I just stalled until I won.

https://youtu.be/_NKKPJh3t6w

Borderlands 2 on PS4 gives you a unique grenade mod called the Contraband Sky Rocket that scales with your level. Every boss dies almost instantly with just one use. The final boss only took two.

In Saint Seiya: The Hades on PS2, you have Lion Aiolia. His basic Cosmo attack is a fast lightning bolt that deals OK damage and forces anything it hits to its knees, meaning you can charge for either more Lightning Bolts, his Special Attack, the Lightning Plasma, which has a good range and can do decent damage, or a Big-Bang, which not only do crazy damage but also stop the opponent from resurrecting if they're KO'd by one. In the same game, there is a game-mode called "Saint Legend", in which you fight a gauntlet of 12 opponents, and 2 more at the end if you won without being killed, and those last 2 are notoriously difficult. Except with Aiolia, who just wrecks everything.

I did cheese Lingering Will in Kingdom Hearts with the Fenrir spin jump strat. Felt a little bad, but hey, I already fought him 100+ times.

Lingering Will is a crime against humanity, so you don't have to feel bad.