The Druid is a monster that appears only in certain dungeons in the west, and does not appear in most scenarios. He has exactly one layer of death protection. This can make it somewhat difficult to kill them with first strikes (particularly for high-damage hitters like the Rogue) but is not going to make them the same kind of daunting foe as the Animated Armor. For the most part, the druid is a relatively weak monster and can be easily killed by a variety of approaches and will only present serious resistance to glass-cannon melee fighters.
One of the most interesting applications of the druid is to use them as fodder for lifesteal. Because they will not die if you attack them, you can lifesteal from a lower-level druid to heal, then allow it to regain its life so you can do so a second time. This approach works with any monster with death protection, but druids have relatively low attack damage compared to other monsters with this special and make relatively good targets. They can also be used to trigger burning effects without killing monsters or to sustain hits in order to activate a dodge using Quicksilver Potion.
If you want to slow the Druid popcorn, always check whether you knocked off its death protection; if you didn't, the 1XP bonus from slow wouldn't take place and you will take damage from it.
Overall, this is a weak monster who you should attempt to prey on for bonus experience. If you need to kill a higher level monster, try to finish it with a fireball so you just need to ignite the burning for the kill.