Elites
Mini bosses are usually present at the end of each Campaigns, Missions and Assassinations.
You can recognize them by their health which is usually much more important than other Enemies.
They spawn on the final wave or just before the final wave.
Their life and name is displayed at the top :
In case the Mini Boss has defense, it's health bar will display like so :
File:Mini Bosses + defense Icone.png
Just beneath the Health bar you can find attributes specific to each Mini Boss.