If you mean Dawn of Souls, no, I never completed the Bestiary. (I could never find WarMech/Death Machine*, or the damn Mythril Golem.) I'm not even going to try getting FFII's Bestiary, which includes the bonus story monsters. ><; I did, however, finish all the Soul of Chaos dungeons, and the Soul of Rebirth. I was hoping there would be some really big dungeon after you defeat Chaos, but it's just the four elemental dungeons. Oh well.
Soul of Rebirth, on the other hand, was pretty difficult. Not just Ultima Weapon (which really just took a really long time and wasn't hard), but you had to go through 'Reverse Pandaemonium' and the normal final dungeon of that game is hard enough as it is without going through it twice.
On Origins, I tried completing the Art Collection (tasty Amano portraits!*), and getting all the items (easy for FFI, very hard for FFII. You might miss some in a dungeon you can never enter again.)
*Yet I run into it on the NES version, every single time I go to fight Tiamat the first time, in every game I play! -_-;
*I loved the 'monster collage'.