Always hitting the strongest 3 units might work, but since part of the reason behind the move would be simple/quick python, just hit the first 3 units in the stack.
I was actually thinking "no python" for the collateral part. The current python means that winning a battle does less damage than losing one. Switching it out for literally collateral damage (as with catapults/cannons) removes the python hog and solves the issue of which units to hit.
From there, we can simply have the postCombatExplode inflict damage on the single enemy unit involved in the combat if the zombie is defeated, which means you still need to select the unit to attack with carefully (as if the combat is close, you'll likely die afterwards in the explosion), but there's no need to artificially separate your stacks and attack one at a time from 2 squares away.
======
Lorewise;
The burning dead shambled through the Bannor camp, trailing chaos in their wake as tents, supplies and warriors alike burst into unholy flame. Despite there only being relatively few of them, they inflicted a heavy toll before the Bannor could drive them back.
[Sheaim attack, collateral damage to several units]
"They cannot be allowed that close again - dispatch swordsmen to slay their hellion sorcerors and end this". Two regiments arrived at the Sheaim temple within the day, to be met by more Pyre Zombies. Even as they hacked down the undead, the fires burst and leapt, clinging to their armour and skin like a living thing. For every Pyre Zombie that was torn down, two Bannor warriors fell back aflame. These creatures truly were abominations...
[Bannor counter attack, units taking damage when defeating the zombies]
======
Mechanic Breakdown
Offense: Standard attack, with Collateral damage affecting
3 units to 100 limit 6 units to 50 limit (as catapults).
Defense: Standard defense.
Defeated in either case:Inflicts 2-20 damage on the unit that defeated it, with no limit (same damage as caused already, but to single unit).
Benefits to the Sheaim (over a standard axeman)
- Cheapest collateral unit in the game (catapults cost 50% more than Pyre Zombies)
- Collateral unit available at an early (and generally needed anyway) tech
- Bonus damage inflicted even when the unit loses.
- Increased collateral damage with further metals (personally I'd trade for the techs)
====
In addition to that, I'd be tempted to grant them a +1 Fire Damage promotion that is AutoAcquired when the player gains Strength of Will (or maybe Arcane Lore) and possibly a +1 Death Damage at Malevolent Designs or Fanatacism. In that way, they do get better as you research lines that are more appropriate to the Sheaim.
====
EDIT:
Effectiveness
Having just tested 20 Modified Pyre Zombies against 100 Champions (both in single stacks) with the Zombies attacking, we had 13 kills and only a single champion "undamaged" (was actually displaying 6.0, so it had taken *some damage*, just not enough to reduce the strength by 0.1). Most survivors were around the strength 3.5 to 4.0 mark, which is fair game to any further Pyre zombies that you may have.
It also took less than 2 seconds to perform using stack attack - the old method was around 10 seconds and would have practically wiped out all of the Champions.
Comparative costs of the 2 stacks
Sheaim: 1200
(60 x 20)
Enemy: 12000
(120 x 100)
Permanent Losses
Sheaim: 17 zombies (1080
lost)
Enemy: 13 champions (1560
lost)
Given how badly the Sheaim army is outclassed there (10 to 1 in terms of cost, ignoring tech cost) - it did very well in terms of damage inflicted. In fairer fights, the battles will tend to favour the Sheaim even more if they are attacking against a single large stack.
Under the old system, even against those odds, the Pyre Zombies would have practically wiped out the enemy (the only thing stopping them from doing so is that they reach a point where the enemy units are so weak that the zombies start *winning* the combats, which significantly slows down the slaughter.)
The original testing here was flawed - I've redone some and retweaked them - the Pyre Zombies will return similar odds to this in a straight fight (equal numbers of Pyre Zombies and Champions), but not in a 10-to-1 outclassed fight as suggested above. They do however become noticeably more dangerous if they have the Sheut Stone promotion.