You can shred almost everyone's magic armor in one fucking turn at level 8+ unless the AP required to move them into position exceeds your allowance.
How many mages do you need for this? And do you mean shred the magic armor of ALL targets in one turn or just one target? You're going to need to be more specific about your example.
At the very least, if I am doing something wrong, it would be helpful to know.
It's not 'how many mages' it's more like 'how much AP do I need to spend, and what resistances am I facing', because depending on your build that may mean one mage or two magi. So I am going in with a few assumptions here; the first assumption is that resistance isn't 30+ ~ immune, the second assumption is that you have decent gear. Third assumption is that you're running elemental affinity to pay the AP tax on your abilities and you have the Pawn. Fourth is that you're being as lazy as possible which means minimal set up. Fifth, it's a close combat playstyle with zero fucks for taking damage.
The BnB for pyro is the following: Dive -> The Pawn (run into the fire) -> Nova -> Ray. Total AP: 5
That alone will pretty much be overkill for a lot of targets, the heavier ones would require you to use more spells like searing daggers, fire whip (super inefficient), fireball to finish the job, and for each one it's AP. A CC spell costs 2 AP.
So, if it's one mage doing it then it has to be a glass cannon/lone wolf build using adrenaline (+2) and possibly flesh sacrifice (+1).
If you're including Source abilities at Max 1 Point, then Time Warp with Fane to buy you more AP to CC a target or position bonus targets.
So, against certain bruisers and other casters, you're looking at BnB + 1 to 2 more AP worth of fire spells and then 2 to throw off any CC.
If we're playing a glass-cannon Fane with 1 Max Source Point the break down looks something like this:
Time Warp (4 AP left) -> Flesh Sacrifice (5 AP left) -> Dive (4 AP Left) -> The Pawn (Into Flames) -> Teleport target (2AP left - scroll or use netherswap) -> Teleport target (0 AP left)
Bonus turn -> Nova (4 AP) -> Adrenaline (6 AP ) -> Reposition with the Pawn (into Flames) -> Ray ( 4AP ) -> Fireball ( 3 AP ) -> Searing Daggers or Ignite (2 AP) -> Medusa's Head ( 0 AP )
If you're going to say "Well, what about having 4AP?" and you want to restrict it to one mage then Rain + Chainlightning. No source? Then you may have to pre-cast a CC spell such as Medusa and then rather than casting Medusa, you'll be using the last 2 AP to nuke and then Medusa Head's petrification aura will CC.