I used to hunt these for fun. Best thing to do is not to engage them as a mob. Try to find out their circle of aggro. The radius is not very big. The area with three of them appear is a good place to learn this.
This is my Mask-Hunter ring set.
1. Bandage - most efficient solo healing ring
2. Hack or Mantis (I prefer Mantis for this. I like the willpower debuff.)
3. Slash(in case they pile on top of each other, slash gets them all)
4. Duct Tape (rather than Scardy) Has a longer ranger than the Mask's attack. Can freeze extra masks so you can take a quick break while they sleep. Also handle extra masks without them flying away. Takes too long for them to come back. DT is like a nice pause button for these buggers.
5. Iron Will - increases your willpower to improve the effects of Duct Tape and increase your own resistance to the Mask's status attacks.
6. Divinity - For luck and stamina
7. Rock Armor or other armors. Your choice.
8. You can put what you want here. I sometimes alternate between Meat and Healing Halo. A passive like Integrity or Fitness should be good too.