Many of the mini bosses move pretty fast. If you have a provoker, provoke the mini-boss onto another creature, not another creature onto the mini-boss, as they probably won't catch up to the boss, whereas the mini-boss will go at the creature you provoke it onto. It also helps to paralyze the mini-bosses.
Also, a lot of these mini-bosses are not spawned from any monsters that are monsters of the day that drop obsidian, it would be good if they were as an incentive to get them to spawn, other than getting the skulls.
Also, a lot of these mini-bosses are not spawned from any monsters that are monsters of the day that drop obsidian, it would be good if they were as an incentive to get them to spawn, other than getting the skulls.