This achievement requires you to kill all bosses - this does not include minibosses. A minimum of 2 playthroughs is required for this achievement. A majority of the bosses are mandatory and the 'Shura' ending has a unique final boss. There are also several late/end-game bosses that can only be fought when NOT doing the 'Shura' ending, as this is the only ending to which you must fully commit prior to the final cutscene. Full list below, lots of spoilers included.
Mandatory bosses are marked plainly as you will encounter them while playing through the game. Bold so there's no guessing required. Missable bosses have a location/conditional description. These are roughly in chronological order.
Gyoubu Oniwa Mandatory
Lady Butterfly (missable - hirata estate)
Genichiro Ashina Mandatory
Guardian Ape Mandatory
Folding Screen Monkeys Mandatory
Corrupted Monk Mandatory
Headless Ape (missable - Ashina Depths in the room immediately after the poison pool, must defeat guardian ape prior to encountering...mortal blade not required for some reason)
Great Shinobi - Owl Mandatory - in all non-Shura endings
Corrupted Monk - True Monk Mandatory - in all non-Shura endings
Divine Dragon Mandatory - in all non-Shura endings
Demon of Hatred (missable - Ashina Outskirts near the original gate pathway idol, only appears when the sculptor leaves after the castle is on fire in the end-game of non-Shura endings)
Sword Saint Isshin Mandatory - in all non-Shura endings
Owl (Father) (Mandatory for Purification ending, can also be encountered, although not mandatory, while doing Immortal Severance and Return endings if criteria for Purification are still met)
Emma, the Gentle Blade + Isshin Ashina Mandatory - in Shura ending only