So here's what I have been thinking for better boss.
First, I agree with the idea of Chaos_Knight, the one to put a either a job tier restriction (or points of rebirth) on certain boss area. This way low levels players could not get the premium book drop from certain boss (Dragon). It's all about if you want to allow players to rush each other or not. ( Your decision)
Second, away to make some higher level boss more a challenge is to introduce chaos damage (as you have probably already thought of). More precisely, bosses should keep some extra strong physical damage in order give agility importance but they should also have a second damage counter for chaos damage (I don't know if that's actually possible to make). This would give healer class way more importance. (There should be more healer class).
Thirdly, I would simply give them a higher hp regen (That would work if possible on when they aren't attacking).
I will try to come up with better ideas.