I think you have to think of the fights somewhat abstractly.
Enmity isn't just a representation of how angry things are with us, but also how well we're covering attacks, deflecting blows, etc. Nidhogg probably does go for the healer in "reality" but with high enough enmity we throw our shield Captain America style at his head and the fire he breathing ends up missing.
Every possibility would be a nightmare to implement though, so we have aggro bars instead .