I made whole bestiary on Skeleton Worrior when I was checking Tyrn on Drefia (I just killed the way). You can find a lot of them near northern entrance and on lower level.
I suggest to kill all low-hp monsters around (to make extra bestiary) and just throw a fire-bombs on Skeletons Warrior respawn :)
EDIT:
The best place to finish Skeleton Warrior bestiary are higher levels on the way to the Undead Dragon boss - Zorvorax during The First Dragon world event!