Cryomancer
Arcane
I agree martials should be buffed but Wizards can avoid damage more easily by staying out of combat range, so they don't need as much HP, and they have good unlimited cantrips so they don't need weapon proficiencies.
Yep. But a single Power Word Kill can end the life of a Wizard with poor hit points (https://roll20.net/compendium/dnd5e/Power Word Kill#content )
Mainly if he doesn't have good CON. Sneak archers too, can end the life of wizards/clerics...
And you can defeat those bosses without access to lvl 7+ spells of course, there are multiclasses and non-spellcasters in the game. I'm also sure it's a matter of damage whether you defeat them and not changing reality.
Not true. We are talking about pure spellcasters here. Of course a multiclass and martial class can defeat then. But not via spells.
Mephistopheles deals a ludicrous amount of damage. And he can dispel your buffs and teleport you to melee range. If you can't cast stop time(and pass the concentration checks), you can't re cast buffs, nor do anything to his resistances/immunities and spell resistance. You NEED stop time to re cast defenses and not insta die. You NEED disjunction to reduce their SR, you need those spells in this encounters. Note that disjunction on P&P is far stronger than on NWN1 and on P&P, it could transform his epic weapon into a mundane weapon.
Multiclasses tends to focus on melee and only use buffs to defeat him.
You clearly wanna nerf casters but probably NEVER played any game as one. D&D already tried to nerf arcane casters to oblivion with 4e and failed miserably. NWN2 too and the most popular gameplay change mod or the game is spell fixes which makes the game more in line with P&P.