That's the problem. But still, I like to develop a party able for that hard fight, and it's a nice option for winning. Sure, it's not the fastest or more efficient way to win.

Sometimes, but I actually never play to "win".  I play to build and play and quest.  I would want the game to continue forever and ever and am often bothered by having to "win".  

i think it's mostly a shortcut to victory when you get carried away by building up and find yourself in a position where you could easily steamroll the whole map, but can't be  bothered to actually do it. i've finished a few games with it. i could have won each of them easily by just making allies of everyone on the map and razing those that would refuse. marching across the whole map razing a few dozen cities takes a lot of time and is fairly boring when your troops outclass them, so it's probably faster to just build a stack of impulsive, bloodthirsty horsemen with pikes and kill the quest boss in the first combat turn before he can even react properly

SPOILER TIPS.

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He has overpower, do no big stacks of troups. Only heroes or strong single units (like storm dragons).

You need a lot of melee damage to compensate for his constant drain health. Since he has high magic resistance he's difficult to damage with spells or even to counterspell the healing. If you're Empire I think you can bring a couple of shadow bolt spammers to lower the resistance.

Remember he's also vulnerable to cutting damage. Again if you're Empire, Curse is much beneficial too, he has really high armor.

At that point of the game the fire breath shouldn't be a problem, everyone should easily have 100% fire resistance, via Nature's Cloak and/or the cheap fire cloaks from shops.

With good armor on your melee, a single life mage spamming wellspring is enough to keep them up.

I have a different means of this, i research refined training, and defense and build some grocers until I have units on par.