We don't know how the Attack & Defense mechanic will work and I'm not sure if even Stardock knows so I got a suggestion on how to do it.

I have played all the Age of Wonders games, Disciples II, Master of Magic and all Heroes game except the first one but I've never thought the combatsystem to be good enough in any of'em. Master of Magic came closest though.

• Age of Wonders - Attack, counterattack. Every point of Attack over Defense (AoD) increases chance to hit (Cth) with 10%. Every point of Defense over Attack (DoA) increases the chance to avoid damage by 10%. Stats are 1-10

• Age of Wonders II & Shadow Magic - Same as above but the stats are 1-20 here so every point of AoD increases Cth by 5% and DoA decreases Cth by 5%

• Disciples II - Every unit got 80% Cth. Some abilities and spells affect this percentage

• Heroes of Might & Magic - Heroes II: Every point of AoD increases damage by 10%. Every point of DoA decreases damage by 5%. Heroes III:  Every point of AoD increases damage by 5%. Every point of DoA decreases damage by 2%

• Master of Magic - Each point of Attack gave each figure in a unit one diceroll with a 30% Cth. If f.e. 3 attacks went through, each enemy figure makes a diceroll for every point of Defense they have with a 30% chance to block the attack

Age of Wonders - This series uses a very simple system that anyone can understand. The downside is that attack & defense is SO powerful and that it is hit or miss. Too luckbased.

Disciples II - The combat is too shallow and also annoyingly luckbased with the 80% Cth. Never liked this. WAY too shallow and annoyingly luckbased

Heroes of Might & Magic - Units always hit so this system therefore isn't applicable to Elemental (I HOPE!)

Master of Magic is the clear winner as the combatsystem have great depth and pretty good predictability while still having that uncertainty of how much you'll lose.

Instead of explaining exactly how the combat works, I quote a combat from the manual of Master of Magic. It explains an interesting battle between wildly different creatures with different abilities. I've bolded the first interesting part:

Melee Combat Example
    During your combat turn, your basilisk unit finds itself starting next
to an enemy unit of elven lords with a regular experience level (i.e., they
have an extra sword and cross icon to supplement their starting
abilities). Note that fantastic (i.e., summoned) units never gain
experience. Both the basilisk and the elven lords are completely
undamaged; they have no enchantments on them and the battlefield is
unenchanted. You are determined to have your basilisk attack those
elven lords for all they’re worth. After placing the cursor over the elven
lords (whom you refer to in a derogatory manner as “Elvis” lords) so that
the crossed-swords icon of melee combat appears, you click on them to
start melee combat.
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     Gaze attacks are resolved before regular melee attacks. Since the
basilisk has the Stoning Gaze ability, this attack is resolved before the
hand-to-hand fighting of the melee attack. Each of the four figures in the
elven lords unit must make a saving throw against your Stoning Gaze
attack or be turned to stone (eliminated). Elven lords have a fantastic
innate resistance ability of 9 (i.e., they are born with nine crosses in
their statistics). The basilisk’s Stoning Gaze has a minus one save
modifier, lowering the elven lords’ resistance to eight. Since the elven
lords are at regular experience level, though, they gain an extra cross,
bringing their total back up to nine. Each cross increases the chance to
resist spells and special magic attacks (such as the basilisk’s Stoning
Gaze) by 10%. With nine crosses, each of the four elven lord figures has
a 90% chance to resist the gaze attack. Unfortunately for the elves, one
of them fails. Its figure is removed from the unit, and the unit’s damage
bar turns green and fills to three quarters of its length (to reflect the loss
of that figure from the group).
     Now both units simultaneously swing at each other in melee
combat—meaning that the results of both their efforts against each
other are applied concurrently (thus, any figure destroyed in this
simultaneous exchange still inflicts whatever damage it can upon the
enemy before being removed from play). Let’s calculate the basilisk’s
attack against the remaining three elven lords first.
     The basilisk has a melee attack strength of 15 (i.e., it has 15 sword
icons on its statistics). Thus, the computer makes 15 die rolls for it,
each with a base 30% chance to hit. With a little luck, the basilisk scores
5 hits from among those die rolls. The elven lords, in response, have a
defense strength of four each (each figure has four shield icons among
its statistics). So, the first elven lord figure steps up to defend against
the basilisk’s 5 incoming hits. The computer rolls four dice (one for each
shield), each with a base 30% to negate a single hit. Unfortunately, it
completely misses and all five hits are scored against that elven figure.
Since each elven lord figure only has three hits (i.e., three heart symbols
on its statistics), it is killed and the two remaining hits are applied
against the next elven lord figure. That figure gets to use its full
complement of shield icons, making four rolls against the same 30%
chance to stop a hit. With better luck than the last figure, it manages to
block one hit, and so suffers the other. Thus, after this melee exchange,
two elven lord figures remain standing in this unit, the foremost of which
has taken a single hit (one of his three heart symbols is darkened).
     Before applying these devastating results to the elven lords,
however, the computer lets them swing back at the basilisk. Each elven
lord has an attack strength (number of sword icons) of six (five for their
starting value, +1 for their troop status of “regulars”). Thus, the three
figures in the elven lords unit throw a total of 18 attack rolls to score
hits on the basilisk. Like all units, elven lords have a base chance to hit
with each attack roll of 30%, but elven lords have a special ability, giving
them a +2 bonus to hit. This increases their chance to hit by 20% (+10%
per bonus point), giving each of their 18 attack rolls a modified chance
to hit of 50%.
     The results for the elven lords are lucky, and they land 13 hits on
the basilisk. For its part, the basilisk has a defense strength (shields) of
four. However, since the elven lords also have the special ability of
armor piercing, creatures trying to block their hits can only use half of
their shields (rounded down). Thus, the basilisk makes its measly two

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defense rolls, each with that base 30% chance to stop a single hit.
Luckless, the basilisk suffers all 13 blows, reducing its full strength of 30
hits (hearts) down to 17. Now, the losses to both units are applied. The
elven lords’ damage bar is glowing yellow and slightly less than half full
(having lost half its figures and with a hit against one of the remaining
ones), while the basilisk’s damage bar gleams yellow but noticeably more
than half full (having 13 damage hits against its 30 total hits).
     With half your unit’s moves remaining (melee and missile attacks
only use one-half of a unit’s moves), you again place the crossed-swords
cursor over the elven lords, figuring that your wounded basilisk can
finish off the two figures that remain standing defiantly against you.
Failing your gaze attack against each of the elven lords again—that
pesky high 90% resistance roll —melee combat quickly ensues.
     Unaffected by injuries to surviving figures (i.e., by any darkened
heart symbols on their statistics), both units attack with full vigor. The
basilisk throws the same 15 attacks rolls (sword icons), each with the
same 30% chance to hit, but scores only three hits this time. The first
elven lord (the one with only two of its three hits remaining) rolls its four
defense rolls (shield icons), each with a 30% chance to negate one hit,
and misses completely. So, two of your three hits are applied to destroy
that figure, while the remaining one meets the last elven lord figure’s
four defense rolls. He manages to block the last hit. The last elven lord
figure is undamaged, but his unit’s strength bar wanes red, showing only
25% of its full strength hit points remain.
     Before suffering those losses, though, the elven lords swing back at
the basilisk. They each roll their six attacks, for a total of 12 throws,
each with the same 50% chance to hit as before. Luck is still with those
swinging elves, for they land another eight hits against your wounded
basilisk. With its two defense rolls, the basilisk manages to block a
single hit, so another seven hits are applied against it, and seven more
of its hearts are darkened. With a total 20 damage, the basilisk now has
only 10 hits left, so its strength bar is colored red and filled to one-third
of its length. The red nubbin graphically symbolizes the amount of
damage your basilisk can still take before dying.
     Note that although the elven lords have First Strike ability, it can only
be used when elven lords are conducting their own attack against
another unit (i.e., during their turn, by expending their own movement
points). The First Strike ability does not apply when units with it are
defending themselves against another player’s melee attack. However,
when the elven lords attack the basilisk (as they would next, if our
example continued), both the Stoning Gaze and First Strike attacks are
conducted simultaneously. Thus, any elven lords that are stoned can
still get in a “parting shot” against the basilisk.
     Of course, any unit that loses 75% of its strength and three of its
four figures might choose to flee rather than to press an attack. A unit
that flees has a 50-50 chance of escaping alive to recover darkened
hearts and “recruit” new figures until it is full strength once more (see
Unit Size and Healing).
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RANGED ATTACKS
    To initiate a ranged attack, the active unit must have a ranged
weapon (missile, magic or rocks, indicated in the active unit window by
a small bow, fireball or rock, respectively, depending on the unit’s
ranged weapon type) and some ammunition. When the active unit still
has ammunition (the combat unit display in the upper right corner helps
in determining this; see Combat), you can click over enemy units to fire
at them. The cursor appears as a small bow over valid targets for a
ranged attack. Note that flying units may be targeted for ranged attack
by nonfliers in adjacent squares. Ranged attacks are resolved in the
same way as melee attacks (see Melee Strength and Defense), but the
target unit may not fight back, and there is a reduced chance to hit a
target at ranges that exceed two squares.
    Most units with ranged weapons have a limited supply of ammunition
for their weapons (see List of All Normal Units and Table J:
Summoned Creatures in the Appendix). When ammunition runs out, a
unit can no longer conduct ranged attacks. Exceptions include many of
the rock throwing creatures and spell casting heroes who can “throw”
ranged magic attacks at a cost of three mana per attack until they run out
of magic power. Note that, unlike other ranged attacks, magic ranged
attacks are not stopped by the Weapon Immunity special ability.
    Both missile and rock ranged attacks lose power at long ranges
unless the unit has the Long Range special ability (see Special Unit
Abilities). As the distance to a target increases, these ranged attacks
suffer penalties to their “to hit” values (losing one “to hit” for every two
map squares, starting with the third square away from the firing unit).
Note that magic ranged attacks do not suffer from any distance penalty.
The effect of distance on a unit’s ability to hit at range is shown in the
following table.
               Distance Penalty for Ranged Attacks
  Distance from Attacker to Target       Percent Base Chance to Hit Target
           1-2 map squares                             30%*
           3-4 map squares                              20%
      5 or more map squares                             10%
    * The base chance to hit is 30% for all units, so this table shows the
distance-dependent penalty for normal units. The chance to hit may be
modified by spells or items. For example, a unit with a +1 to hit bonus
always has an increased chance to hit of 10% (as long as no other to hit
modifiers are operating). This means that the unit has a 40% chance to hit
at one to two squares and a 10% chance to hit at five or more squares.

It's a little hard on the eyes I realized, but find the manual and go to page 93.