I have discovered some valuable information from farming the Plunder Skulls during the Witches Festival and I thought I would share it with you guys so you can get more Skulls per hour. Here is what I have learned:
1) Defeating ANY Bosses (World, Delve, Dungeon, Trial, Dark Anchor, Daedra things the fall from the sky in specific locations, etc) to obtain skulls.
2) Skulls have a 5 minute cooldown so you CANNOT get more than one every 5 minutes.
3) World Bosses and Delve Bosses have 10 minute cooldowns on loot, so you will NOT get more than one every 10 minutes if you only farm World Bosses or only farm Delve Bosses. You must alternate between various types if you wish to maximize your farming.
4) If you farm in Group Dungeons, be sure to wait 5 mins between Boss kills to maximize your runs.
The only exceptions to this that I have not tested is if logging out and in resets either the World Boss/Delve Boss cooldowns or the Plunder Skull cooldowns.
Happy Farming!
Cas