The setting is low fantasy medieval. Magic (and other supernatural things) belong exclusively to the realms of the gods, and are not for mortal beings to wield. However, there is one major exception.
Each and every recognized kingdom in the world has a single major patron deity who watches over it. They may have other lesser minor deities as part of their pantheon, absorbed from conquered cultures, but the most important thing is the blessing of the patron deity.
Each kingdom has possession of a suit of divine armor and a matching divine sword, bestowed upon them by their patron deity. Anyone is capable of wearing and wielding these artifacts of power, but it requires an uninterrupted hour and a team of high-ranking priests of that deity to perform the prerequisite donning rituals in a sanctified location like a church or cathedral. Part of the donning ritual includes the armor and sword being magically resized to suit the kingdom's chosen champion.
The armor has the appearance of a full suit of medieval plate mail, but with the metal taking on a shining golden color, rather than having the appearance of steel. The sword is generally a knightly arming sword with a similar appearance. Exact styles vary, and are up to the individual patron deity.
When successfully donned, the inside of the armor is supernaturally connected to the patron deity's private god-realm. The chosen champion is technically no longer inside the armor at all, although they appear to be. They are fully able to control the armor as if they were wearing it, and have the same vision, hearing, and nearly the sense of touch as if they were wearing mundane armor.
The effects of the divine armor are as follows:
- The armor itself is absolutely invulnerable to damage in any form. Projectiles cannot pierce it, blunt impact cannot dent it, fire cannot scorch it, etc.
- Light and sound are transferred to the wearer's senses normally.
- Force only to a non-harmful degree is transferred to the wearer's senses. This means they can lift an object and feel its heft, touch and feel solid objects, and so on, but will never be harmed by a mace to the head. The impact will be reduced in strength when transferred, so the wearer would feel it only dully.
- The wearer is aware of the size and shape of the armor, but does not feel the weight of the armor. It is sort of like wearing armor made of styrofoam or cardboard: all bulk and no weight.
- Temperature and particulates are not transferred. The wearer always enjoys breathing and existing in the pure and comfortable air of the god-realm, which means they can stroll through clouds of poisonous smoke and burning buildings with nary a care. This has the side effect of cutting off the user's sense of smell and taste.
- Nothing else can enter the armor with the user unless it too goes through the lengthy prerequisite rituals, and each patron deity's is unique. That means no daggers through the eye holes or armpit slots. They are simply blocked by divine force.
- Things do not stick to the armor. That means tar, blood, and boiling oil will just slide right off onto the ground with no residue, leaving the armor always impeccably clean.
- The armor constantly glows with golden light. This is sufficient to see by in an otherwise lightless place. You can easily see a chosen champion from a mile away in any direction, especially at night.
- Assume bodily waste products are handled automagically without the wearer needing to remove the armor. I don't want to dwell on this topic.
The effects of the divine sword are as follows:
- It is just as indestructible as the armor.
- It can be commanded to instantly return to the wielder's hand, so it cannot be lost or disarmed.
- It is razor-sharp and never loses its edge. Furthermore, any material with which it comes in contact is temporarily magically softened, reducing the required force to cut through that material. Mundane steel armor could be sliced through as if it were made of leather, leather armor could be cut through as if it were simple clothing, and so on. With a little elbow grease, it wouldn't be impossible for the wielder to carve their own path clean through a solid stone wall with just human effort and their sword.
- When striking at a living foe, the sword shifts itself (and the wielder's body) to actively target the enemy's vital points. The vast majority of injuries caused by this weapon are therefore fatal, and typically fatal in a single blow. It would be extremely difficult to only lose a limb to this weapon.
Between these two pieces of equipment, the chosen champion of a kingdom can be considered essentially invincible.
However, there are a few important things to note:
- The champion is still human inside all that divine power. They will still experience stress, can be tricked, etc.
- One person, no matter how invincible, is still just one person. A group of enemies can split up and the champion is only physically capable of pursuing one of them.
- The champion still becomes hungry and thirsty at a normal rate. Food and beverages can be supplied as part of the donning rituals in order to allow the champion to later consume them while still wearing the armor. Further rituals can additionally be performed after the fact for this purpose as well. Otherwise, the champion must remove the armor first, which requires a full donning ritual to get back on. This can make participation in lengthy campaigns difficult.
- The champion still becomes tired at a normal rate for the strenuous exercise they are doing. This is partially alleviated by the weightlessness of the armor, but combat is still a tiring activity. They can also comfortably sleep in the armor if they so choose, as their real body is able to relax in their patron's god-realm. (The armor won't stop glowing though.)
- The champion is not protected from very loud noises or bright lights shined into their eyes.
- The champion can be trapped in something like a deep hole, which doesn't rely on direct harm and can't be easily escaped by cutting through something.
- If the divine sword of one kingdom's deity strikes the divine armor of another kingdom's deity, both champions are instantly transported back to the location they most recently performed the donning rituals. This is part of the treaty of the gods, agreeing not to use divine power to directly combat each other.
- If the champion who is wearing the armor and wielding the sword dies, e.g. by dehydration after sitting in a pitfall trap for a while, the corpse and divine artifacts are transported together back to the previous donning ritual location in the same manner as above.
So the question is:
Given each kingdom is always able to field one single incredibly powerful individual, whose supernatural capabilities and limitations are common knowledge, what would be the prevailing tactics in a war between two kingdoms, assuming neither side has any other significant advantages or disadvantages?
In other words, how would rational kings with vast resources and armies of mundane soldiers tactically make most effective use of their single invincible champion, while minimizing the impact of the enemy kingdom's champion?
Edit: While I appreciate the strategic insights, I am really looking for tactics, not grand strategy. That means I am trying to find an answer to things like:
- What kind of supplies/armaments/formations/other tricks might a medieval army bring to a battlefield, in order to best deal with the enemy possibly having an invincible champion on the other side? i.e. It is common knowledge that the enemy's champion is unbeatable in direct combat, but can be outmaneuvered and possibly trapped. What are effective ways to do this?
- What kind of supplies/armaments/formations/other tricks might a medieval army bring to a battlefield in order to best support their own invincible champion if they have chosen to deploy theirs? i.e. How can our side best protect our own champion from being outmaneuvered and possibly trapped?