So the sheeps become invulnerable for few seconds when you stun them during the preparation for Dream Bubble... 0 damage when it doesn't have any HP left or maybe it has 1 HP.

Platform: Windows 7
ISP: DNA Welho
Type of Internet Connection: ADSL
Internet Connection Speed: 24/1
Date & Time: 21:00 EET
Frequency: Every time if you do the steps correctly
Character Name: Tera Ayamia
Race: Miqo'te
World: Kashuan
Class/Skill Level: Lancer 30
Area and Coordinates: Coerthas, 48,33
Party or Solo: Duo
Monster Name: Brutal Sheep
In-Game Time: No idea D:
Steps:
1. Brutal Sheep does it's sleep ability
2. Stun the sheep with Leg Sweep when you wake up if Dream Bubble haven't been used yet
3. Try to damage the sheep, you can damage it until it's at 0-1 HP then everything will do 0 damage
4. It can't be killed until it uses Dream Bubble. Then it's vulnerable again