Despite the achievement description, you don't have to defeat any event bosses (except Moon Lord), the Empress of Light, Deerclops and Queen Slime. Both world evil bosses need to be defeated (Brain of Cthulhu, Eater of Worlds).
While you can get the other world evil seeds from the Dryad in a Graveyard, it is easier to make a new world to fight the other boss.
Playing the game regularly to Moon Lord does make you fight the majority of bosses but there are still optionals that have to be defeated:
King Slime has 3 spawn methods, killing 150 slimes during a slime rain, using a Slime Crown (crafted using 99 gel and a platinum/gold crown) and a 1% chance to spawn near the ocean during the day if grass is available above the depth 0.
Queen Bee can be summoned by breaking the Larva found in giant bee hives in the Underground Jungle, or by using an Abeemination (5 Honey Blocks, 1 Stinger, 5 Hive, 1 Bottled Honey).
Eye of Cthulhu has a 1/3 chance to spawn if its night, the boss has not yet been defeated, 4 Town NPCs (including Travelling Merchant and Town Pets/Slimes) are alive, you have 200 or more max health and more than 10 defence. The Eye of Cthulhu can also be summoned using a Suspicious Looking Eye (6 lenses, can be found in Gold Chests).
Duke Fishron is summoned by fishing in the ocean if you use a Truffle Worm as bait. Truffle Worms are a rare critter in the Glowing Mushroom biomes an can be difficult to catch as they burrow when startled. The easiest way to stop that is to use an invisibility potion and to dash while swinging a bug net in it's direction.