Rhys how hard to you want bosses.
1. High HP - Decent Damage. (Tank and Spank)
2. 2 Bosses - 1 Heals 1 DPS's (Teamwork)
3. A boss that requires strats, more then decent team work, a sense of randomness. high levels, better then average players and synergy between elements.
Remember these are just general difficulty scales.
If you want a boss that is equal to number 3, say so.
I could have some real fun if you want bosses to be equal to the number 3 difficulty range.