Just self restrict yourself from using those tactics, and find a different way to win with that faction. For example, I self restrict myself from using beastmaster during the early game.

First off, I really cannot say that I'm particularly concerned about the balance of custom factions (or any lack thereof). It is very easy to create a faction which is significantly stronger than the default ones, just as it's easy to create a custom sovereign who is much better than the default sovereigns.

Also, Shadowbolt costs 18 mana before reductions, not 8 (although you might be able to get it down to 8, since just by being a Mage you get a 25% reduction in tactical spell costs, and you can improve that with Affinity and some items). Infection, however, does only cost 8 mana. Nor will casting Shadowbolt on a target guarantee that any spell takes effect - while it will make it more likely to take effect, there is a minimum resistance chance somewhere around 5% even for targets with no spell resistance, and trained units typically lack the spell mastery to push it to that minimum chance even if they are facing a zero-resistance target.

I have said it before in other threads about these kinds of things, but if something isn't fun in a game, just don't do it. Nothing in the game is forcing you to do anything, and it's not like this trick with infection is particularly different from just going in with a number of undead units at least equal in size to the army you're facing and terrorizing all but the weakest, then killing off the enemies one-by-one to slowly free up units from terror duty so that they can slowly dogpile frightened enemies. Incidentally, a Mage is the best champion type to support that kind of strategy, too, because a good damage Mage can kill (or nearly kill) one or more units per cast, while an undead summoner Mage (namely, your Sovereign, since good luck getting another undead champion) can summon up an extra unit and then go terrorize something, and that one summoned unit can be the one unit beyond the enemy army size which slowly whittles down that first terrorized enemy. Infection just allows you to do the same thing, but it makes it faster because you can have just one or two units on Terror duty while everyone else beats down the enemy.

Mage are super-powerful when leveled yes. As a custom race that has quest maps, and uses a warlock "Men" hero, I was up to level 20 in no-time.  I have Initiative of 55 and -1 casting time so every fight starts with me clicking Horrific Wail and it immediately being cast (I even get the turn after that, effectively 3 turns in  row when a fight starts).

Horrific wail hits for 173 every time.  The only thing that doesn't immediately die are magic immune creatures - which I can kill with thunder-strike teleport because it has no immunity check like shadowbolt.

But I caution against assuming mages are the most powerful, I'd argue defenders are right up there.  A defender with Discipline, and some good equipment/traits is a beast and trivializes the games even more so than mages.  In about half the time or even less than that when compared to the mage above, I had a defender with around 60 attack, 2 counter attacks, heal and first aid. His defence was around 90, and spell resist was 98.

It was even easier than the mage because I could auto-resolve every battle.  Throw in a few knight units and I would auto resolve with less than 10 hp lost when fighting deadly/epic stacks. I could never do that with a mage who had 50+ dodge, mana shield, instant-map-clear horrific wail and mana-blasts for 15,000+ (yes, 15 thousand and higher) by the end of the game.

yeah...

this strat requir0e TWO mages, which cast several spells each... and they have a bunch of tanks too...

i think it doesnt exist a combination of 2 mages casting several spoells while being covered by tanks that doesnt kill any enemy

It does!! How can some mouth breathing neck beard be superior to my super smart gets-all-the-girls dress-wearing pew-pewer!?!?! SO MAD!

I actually love the different experience... I tried more recently with that super-crit assassin type, the only problem is magic and high accuracy. The assassin has around 115 dodge (lucky, assassin gear, buffs, etc) but the odd champion or buffed unit will hit him (playing on challenging) and then he loses half his health. Or God forbid someone hits him with a spell! CRUMPLE

I love this game. Need more DLC pls - maybe a big one. 29.99 for way more spells, items, updated skill trees, faction/hero traits (so that rogues aren't using shields? ) maps, quests etc?