Why? did you die for a mage?
mage is stronger in some bosses because the boss's pdef are bigger than mdef, simply because of that. the combated titan is doing less damage than the mage, precisely due to pdef and medef.
now let's see player vs player,
mage pdef 3.5k ~ 4.5k
fighter patk 2.5k (tanker) ~ 6.5k + (duelist, gk, titan) not counting archers, db, etc ... if the mage fails to hit the dbuffs he won't be alive for more than 4 hits. no matter how much the matack varies between 14k ~ 21k the magician doesn?t have enough hp to take more than 4, 5 hits ...
debuffs exist for a reason: slow the target, paralize the target, removes buffs, weaker the target all this is to give a chance for the mage kill your opponent.