Can I complete 100% bestiary on my first planet, no matter my seed?

Solution 1:

You can always complete the Bestiary, but you will also need enemies from the other type of evil. This includes the evil-specific bosses, the Eater of Worlds and the Brain of Cthulhu.

You can accomplish this by taking some Ebonstone or Crimstone from a different world, or by purchasing the appropriate seeds from the Dryad. Once in hardmode, the Dryad will sell seeds for the evil that is not native to your world if she's in a graveyard biome, created by placing six or more tombstones close to each other.

Some NPCs and enemies require the ingame Halloween or Christmas events in order to spawn, which are normally only available in accordance to your system's clock. It is possible, however, to trigger a single ingame day of Halloween or Christmas by defeating the Pumpkin Moon or the Frost Moon respectively.

The entries for the only two missable enemies, the Dungeon Guardian and the Tortured Soul, can be obtained by talking to the Clothier and Tax Collector NPCs respectively. Every other enemy that was missable prior to 1.4 (due to being exclusive to worlds pre-hardmode) can now be found in hard mode as well.