Help with mission

I can't seem to recall the name of the mission, but it's the one with the dragon flying into the whirlpool and then you face him in the tower.

I can make it into the whirlpool just fine, but once he's up int he tower, I get him to about 2/3 health and he uses a skill that gives everyone -10 degen.

How do I counter this? I can never kill him before the degen takes its toll, and you can't interrupt his skills. Help me out?

Mission is called uh, Harvest Temple or Unwaking Waters. Move out of range of the level 10 things Kuunavang sends out? They are what causes the degen, or use AoE to knock them out. Once up on the tower, Kuunavang generally sends them out in a direct line in front of her. Just move to the right or left and you shouldn't be in range anymore, if you're a monk, you can heal your team from the sides. If not then just move behind Kuunavang and attack her from there. Heal party might be a good idea here.

Best of luck.