For those who have heard of supercombattants, are anxious to try one, but have had trouble making one work, I present possibly the simplest and most fail-safe of all such critters: the Vampire Queen.
She's available to most, if not all, nations, and requires very little to get started. All you'll need are two levels of Alteration, and your Queen will be ready to rock! Design her with 3 in Earth, and at least one in Water. Whether you augment the Death and/or Blood that she comes with naturally is up to you, but not necessary. Reasearch Alteration to level 2. Set your Queen on the back of the battle board. Don't give her any troops - not even if she's managed to generate a Vampire while you've been researching. The point of this is that she can take provinces on her own, freeing up your other troops to take other provinces. Have her cast Quickness and Stoneskin, then attack reamost enemies. Quickness will mean she attacks twice per round, and Stoneskin covers her only real weakness, which is a lack of protection. She flies - she'll reach the enemy immediately. Her attacks are life-stealing - meaning that they both reinvigorate and add hit points to her when she hits, so the fatigue she incurred casting the two spells will be gone or much reduced as soon as she scores her first hit. She's etherial - very hard to hit. If she is hit, she regenerates, and has the life-stealing attacks already mentioned.
Choose a province with light troops in it for your first couple of kills, and do make sure that the province is in your dominion. Once the Queen has a few province kills under her belt, she'll gain enough experience to be able to take on tougher foes. If you do miscalculate and send her into a province she can't cope with, no problem! She reappears in the home citadel the next turn, ready to venture forth again.
It is probably worthwhile to give her two in Water, so that once you've researched Enchantment 1 you can add Breath of Winter to her spell list, but I have found she does fine without. You will also want to replace Stoneskin with Ironskin once Alteration 3 is reached, and Ironskin with Invulnerability once you reach Alteration 5. This Last is the reason for the 3 in Earth, but you could simply give her 1 Earth if you find yourself short of design points. For the adventurous, a level of Astral magic will allow you to add spells like Personal Luck and Astral Shield to her repertoire, making her even more powerful. A strong dominion is another design consideration when comtemplating using a VQ - the farther and faster your dominion spreads, the more choices of province to attack your Queen will have.
One Last note - among all her other attributes, the Queen is also stealthy - make sure you don't have her sneak into provinces you mean to attack