Percent HP attacks are a major problem that almost all high level bosses share. In essence they allow a thief to tank as many hits as a warrior regardless of their defences and max HP. However reverting these attacks to dealing flat damage is not a solution because it is impossible to strike a balance between a thief's meagre 10k hp and low defence and a warrior's 30k with high defence. A boss that has already attempted this balance is Hilla's pet Blackheart in the Dimension Invasion party quest. It has attacks damaging enough to instantly kill some classes whilst warriors brush off the damage without a worry.
The following solution retains the ideology that squishy (low defence) classes should die in fewer hits than tank classes as seen with the flat damage attacks, but it takes it one step further and gives tanks a way to "shield" squishy players from damage.
Here's how it works.
Bosses deal flat damage with all of their attacks, for example Gollux's purple breath attack might deal 20k damage, but the damage is split across all players hit by the attack based on their maximum HP before defence is factored in. Using the Gollux example, if player A with 10k hp and player B with 30k hp are hit with the purple breath attack then player A will take 5k damage before defence is applied and player B will take 15k damage before defence is applied. Sounds pretty close to a 50% hp attack, but defence would actually have meaning again. If there were 3 players fighting the boss then their damage taken would be reduced further if all 3 of them were hit by an attack.
Note: When I say defence I also mean skills that reduce damage taken such as "Iron Body".
On the other hand if player A managed to dodge the attack but player B didn't then player B would take the full 20k damage. With that in mind I think certain classes built specifically as tanks (Paladin, Maplestory Battle Mage, Mihile) should have skills that allow them to take a portion of the damage regardless of whether they were actually hit with the attack or not. They probably shouldn't take as much as if they were actually hit, that gives incentive to stay as a group and "body block" the boss.
With this system in place I think it's only fair that the chaos/hard mode of bosses dish out significantly more damage, to the point where grouping up with 3 or 4 people is mandatory, not optional. It would make fighting these hard bosses more party focused, rather than it simply being 2 or 3 players soloing the boss at the same time. I'm trying to get rid of the "every player for themselves" party play and replace it with real party play.