Vex Thal access quest

[Sian]

there is only one possible spawn for prison guard (but he has PH). Your best chance to get the shard is the other spawns (blood guards, death guards). Honestly, it's not too long to get the shard. The 2 other kind of mobs I named can pop at the place of most any other so you can have round with over 10 mobs that can drop the shard.
Ssra is harder because of the repop which is longer.