Ehh, I suspect most of that comes down to specifics and counters. A medic will stop being effective if they invest in one-hit kills or something. We could get a similar effect with a mobile golem factory deploying replacements, but then that is subject to long-range strikes against a large, low-mobility target. Skirmishers have their own issues. Armoured artillery is reliable, but can be out-competed by dedicated artillery. Flying tanks tend to be slow and easy-pickings for fast anti-air. Light infantry is similar to skirmishers, a difficult point between lacking the stopping power to defend against cavalry, so needing support, but being too specific to work well in a formation. They can be great to have, but it takes effort to make use of them, and some sort of cavalry with a retreat feature, like teleportation, would be terrifying against them...
But it is still a very good approach to take prior to meeting the enemy.
Hrmm...
Well a heavy infantry to anchor formations is a nice basis. This may turn into trench warfare or something so it would be good if it was flexible enough to do something other than stand and be a big target, but magic armours can get pretty impressive...
I would love a dedicated reinforcement unit. Something very versatile. Able to modify itself in the field to provide whatever is needed, be it armour-penetration against a boss, mobility against something with devastating attacks, armour against something with area attacks, precise ranged weapons against aerial threats... I like amorphs for this, but some sort of golem with an assortment of attachments, or cavalry with very diverse weapon training and a choice between fast but vulnerable mounts or slow but secure dismounted combat...
Artillery is a given. The ability for long range to concentrate fire is massive, and a dedicated unit can pack massive ordnance.
Antiaircraft is going to be necessary. Not necessarily dedicated, but our current fire wands don't seem suited to the task so it would need a big investment in ranged armaments or something specialised like air-burst-homing. Or we could try to deny the airspace by producing clouds of acidic steam that loiter over our battles.
Numbers are also valuable. With a heavy anchor infantry we would want an expendable filler. A mobile golem factory spitting out weapons on legs could do it. Undead are always fun. Or we could try something with an insane gestation rate that injects eggs into enemies...
Assassins and scouts are important. Some sort of moving cloud of shadow with a sealed core of etching darkness that it can use as a suicide attack but generally avoids combat and concentrates on scouting?
Archers... Well we already have those fire wands. I feel as though artillery can do much of what archers should, but massed ranged attacks can keep air opponents at bay so long as they can't be out-ranged and are devastating against an enemy approach, especially against morale.
I consider this a draft and hope it more inspires than is supported...