What mixed hero class of the Haven faction do you like?

The Battle Mage is the most useless...
and at maximum level, you still end up with either an Archmage or a Warlord.

This ability is useless, but the class itself is quite good. By acquiring the War and Resistance skills (neither of which require Melee or Archery), this mage becomes a real pain in... any part of the body. And, to be fair, the +20% effects only affect a couple of combat spells and illusions, while the hallmark of this magic lies in completely different things. But sometimes, even these skills come in handy.
The Archmage is, of course, the "ideal" for any mage in campaigns and on epic maps, but in the case of the Order mage, it will primarily enhance the additional schools (which requires developing these schools at a high level). If they are taken "opportunistically," then even the adjacent "Monk" and "Shadow Mage" will be better in certain situations, as they provide simple but permanent spells "built into the hero." And the combination itself, besides being "supported by magic," is quite meaningful, since both Order magics complement each other very well.
Regarding combinations with opposing schools:
The Enchanter is a class for a druid raised in the Knight's Seminary (or a mage retrained as a druid there). To have both sand, slow, and good summons, and to learn everything in the Guild.
The Sorcerer is a pleasant and useful bonus, but Chaos is taken exclusively for the "checkbox," which puts an end to the possibility of developing War skills (it will override "Battle Mage," and you will be left with neither here nor there with Chaos magic). Starting in the Fortress of Death with access to one of the two schools, or capturing another's fortress with the same access, makes this career more meaningful. But the bonus is excellent and worth considering.
The Illusionist, Seer, and Warlock have useful bonuses and are chosen based on your needs (the last two, as mentioned before, are usually easily acquired through the University, as are the Monk and Shadow Mage), if you don't want to spread your forces to an adjacent magic.

Yes, it's better to summon more clones; that's what his "ability" is for, as far as I remember.

If a person chooses to focus on creating clones, then the Illusionist is definitely better than the Archmage tactically. The thing is, this is good against a "Gray Player" (not counting Djinns or something), since he doesn't know how to remove/cancel spells.