I think two changes that probably need to be made anyway would indirectly solve this. A: Re-terrain the boss areas.
Basically, for bosses that have cliffs/platforms/random things near them that allow PR/Ele/etc to safe-kill the bosses, said places need to be reworked so as to disallow such cheesing.
B: Agility's effect on armor.
Right now, it's one point of armor per 33 AGI, which generally gives most classes enough durability to tank fairly reasonably. Items like the Ring of Struggle, which basically entirely negates the need for a tank with its +50,000 regen (especially if you have multiple, since the regen stacks fully) make tanks pretty pointless for most of the Chaotic Realm's creeps. If agility is to provide armor in a game where non-agility heroes are supposed to provide a primary tanking role, it probably shouldn't scale well enough to allow non-tanking classes to tank as well as they do now, save for with extreme level discrepancies (a level 999 MR being able to tank Horrors would probbaly be okay). I notice that this is a problem with even non-AGI heroes due to allstats items like the Ring of Struggle, Thanatos' Wings, Ring of Existence, Lexium Crystal, etc, as these items still give enough armor to allow a hero to take a few hits and kite the enemy (again, barring any need for a tank).
Granted, I suppose fixing AGI/armor scaling still wouldn't really fix the fact that the armor bonus from chaotic creep/set items still isn't bad, and I can't offer a more elegant solution at this time beyond simply tweaking numbers in those cases. The changes above would force heroes to take hits to kill bosses, and if taking hits actually means something, then tanks and healers will become important.
