If you noticed, divine armor did have a weakness (it took full damage from chaos). This means that no armor type by default reduces all damage by an amount.
If you do something like -75% all damage for a boss unit, it defeats the purpose of armor types at all (you mithers well make it take full damage but have 4 times the health for easier DPS calculations). If you want bosses to take the same damage from all attack types, then give them an all round armor which has 0% reduction for all attack types meaning nothing is super effective against them nor super weak.
Traditionally, bosses take reduced damage or are immune to strong moves (instant death, % health reduction, stuns) and have shortened negative buff duration or effect. Bosses may also be resistant to slow from frost attacks and poision to prevent compounding slow. Bosses should avoid being immune to magic or immune to phyiscal unless that is their purpose, I have seen a lot of bad maps make every boss immune to magic late game which defeats the purpose of even giving the player spells to cast on enemies.