Early game, I do have access to tier 2, but it has come to the point where all enemy capitals are guarded by minotaur guards. They are nasty and there are 6 of them. They kill my level 30 fighter no problem. They cut through teir 2 troops like butter. They have all around attacks, so numbers are not a problem.

How does one deal with them?

Best I have come up with is sending in a high level mage or fighter, doing as much damage as possible, then immediatly sending another high level hero. This seems dumb that I have to lose to win, so to speak.

Is your level 30 fighter a pure fighter?

A level 30 chieftain can kill 6 or even 8 minotaurs by himself.

You just need the right spell.
2 vampirism spell with round attack should be enough.
For the 4 level 1 spell slots, you can have 2 magic amor and 2 astral energy.

Of course, you should have decent gear.
You should be able to get those gear from the shops in your stronghold.
It helps to have a weapon with first strike although it is not necessary.

If you don't have access to tier 2 spell yet, it will be helpful to have first strike.

You just position your fighter at a corner where only 2 minotaurs can attack you.
With first strike, the minotaurs that has little hit points left will not attack you.
So you will be able to spend a few turns to recover your hit points, then you can finish off
one minotaur with low hit points and take on the next minotaur.

T2 only:
a)
1. Deadly terror ritual
2. Sniper + morale diversion
3. Sorcerers - vulnerability
4. Hero + Sorcerers Fear spell
Deadly terror drops pretty often, you can hire sorcerers as mercenaries if you don't want to build them
b)
1. Ghoul or Slug
2. Phantom form on it.
Not only T2:
a)
1. Deadly Terror
2. Cloud of Terror
b)
1. Mass suicide
2. Mass suicide
c)
1. Pestilence (T4 ritual, -50% hp)
2. Finish them with anything.

Thanks guys. As Gamexiao said the key was using a first strike weapon. What a difference it made. I took the 30 lv Chieftain and downgraded his weapon from the sword of insanity to a guards halberd and he had no problems. Well it was a bit of a closer fight than I like, but he won.

Now I have run into the same problem with the vampire and ghost guards. :(

Which units/magic schools do you use/have?

Max level pure fighter, tier 2 units and spells. Got the 'lawful' units. monk, guards and ballista. I also have fairly high level of every other class.

Level up guards+monk, pick sniper.

Stand in corner:
Monk,sniper, guard
guard, guard

One doubleshot must nearly kill vampire, finish with guard.
Then cast fire weapon and kill ghosts one by one. With good crossbow you would kill them without fire weapon at all.
other units insignificant.

This works pretty well. Also, don't forget that the monk has the inspiration spell. In addition to morale it also restores 2 stamina, which means more uses of doubleshot.