I can't help doubting any of these deserve to be called great accomplishments, seeing how I've never done speedruns or major high scores, yet several occasions of managing to defeat a Megaten boss with some real flair come to mind.... with SMT Nocturne Hard Mode I came close to killing Matador, Unofficial Mascot of Wake-Up Call Bosses on my first attempt, second was the charm. May not sound like much but I recall it taking most other players many more tries.

On a more clearly remembered, and hilarious, note I made Polaris of Devil Survivor 2 King-Maker Route practically kill himself for me in phase 3 of the fight. After rendering his Bounty Spear limb helpless via spawncamp, I approach his main body with Repel Phys and Victory Cry on the Protagonist and two Repel Phys modified demons. Then just sat there tanking his Wave Motion Gun with my face, healing/regenerating, and watching Cepheid backfire until a reflected volley ended Polaris.

Lucifer in SMT IV suffered a similar trick. A dialogue tree comes up after casting Tetrakarn in my party, so I pick the choice I know will piss him off. After responding to his question of who I fight for as a Chaos aligned person would, The Morning Star flips his trout on my lawful(I am not proud of this alignment) ass, gains two extra press turns... and immediately blows himself up when his Hades Blast bounced off my Tetrakarn. Lucifer's turn automatically ends and gives my entire party gains Smirk status... We proceed to double the damage Lucifer inflicted on himself. It was glorious.