As a general guideline, for melee champions, I would like to see them balance approximately to a normal unit trained from a city. In the beginning of the game, that's 3 club-wielding peasants. At the end-game, it's 9 heavily armoured knights.
That sounds all right to me, as long as there are more than 2 options for hero development; melee champion and archmage (or 4 if you really want to stretch it, with ranged warrior and 'just tagging along for the xp'-governor being the other options).
The guy you describe is your standard tank; capable of engaging and surviving a battle with a trained unit that is level appropriate. But he shouldn't be able to quickly dispatch of these guys, and not without taking some damage himself. And if he's engaged by several units he should be in a lot of trouble.
The archmage hero should be in the back of the field, buffing, debuffing, and casting direct damage spells. He should be kept out of melee combat, as he won't survive for long in it.
You could have a hybrid, the battle mage. But this should come at the expense of casting power or spell diversity, and some combat power. This guy is in the frontlines doing direct damage or debuffs, either killing units outright or letting other units finish them off easily. But in single combat without any spells this guy wouldn't be as good as the tank above.
(A Sauron type hero, both powerful in spellcasting and in combat, should be possible. But only once in a while, not every hero should become this powerful or the fun will soon wear off.)
Assassin type heroes could be specialised in taking out single targets, sneaking up on the archmage or stabbing the tank in the back. Or they could be specialised in taking out groups of troops quickly. Imagine a Witcher-type hero running up to a group of soldiers, throwing a smoke bomb to hide his approach, knocking some troops down with a magic sign, cutting down those left standing in a few swooping motions with his greatsword, and then quickly retreating before nearby troops have time to react.
In either case, this type of hero should not stay around and fight troops in straight combat. And if the attack fails he should be in BIG trouble, so you'd better have some way of bailing him out.