Okay, let's see... We're going with 11 Drain Dice, I assume 10 physical boxes and a Force 13 spirit? So the spirit rolls 13 dice, the spellcaster 15... I'd run a program to check out the odds at different services levels, but let's just look at the drain. I'm using AnyDice for this, so anyone can doublecheck the math if they want to.
------------
So I need the higher of 0 and the drain resist, which would be 2x(13d{0,0,1}) - 11d{0,0,1}... 3/4 of 3P or more, 4/9 of 6P or more, 3/17 of 9P or more, 1/9 of going k.o., very minimalistic chance at dying. The Mage will really take some pain on this, making them rather vulnerable for enemy gunfire, and they got a fair but low chance at the Spirit berserking instead.
AnyDice: output [highest of 0 and (2*(13d{0,0,1}) - 11d{0,0,1})]
------------
Now let's add Edge. Assume the spellcaster has less than 5 Edge so he uses rerolls for both his conjuring and his drain resistance. The spirit has an Edge of 7 on a dicepool of 13, so he'd be better off exploding (average 8 hits instead of 7.22). So let's take 20 exploding dice *2P damage vs 11 rerolled dice on the drain resist. Averages calculated beforehand bring us to 10P but let's actually check the math.
I also need to make sure AnyDice only explodes the 6s, which is done by using 100 and 101 instead.
AnyDice: set "explode depth" to 10
output [highest of 0 and (2*(20d [explode d{0,0,0,0,100,101}]) - 11d{0,0,100,0,0,100,100,100,100})]
This gives me an average of 10002.78 which pretty much translates to about an average of 10P. I assume the Mage has Body 4, but let's be nice and assume Body 5. So 11P is K.O. and summoning failed, 17P is dead. In other words, any value at 1100+ is a failed summoning and any value at 1700+ is a dead summoner.
4/7 on not going K.O. 3/7 on going K.O. or worse. And keep in mind going K.O. means the very powerful spirit gets to break free and turn against the arrogant bastard's party. A bit less than 1/7 chance of dying. So in other words: 4/7 success, 2/7 failure, 1/7 death. And that is after consuming 2 Edge.
At Body 4 the odds were 50% success, 21% dying, 29% failure by the way.
------------
Now let's toss our Body 5 Mage against a Spirit of merely Force 10. That's 15 exploding dice. Success chances are now about 9/11, dying chance is a mere 1/35th. But a Spirit of this level is doable (though heavy) for enemies, it still fails ~20% of the time (18% but who's counting), the dying chances are not THAT bad (10 of these summons only are 1/4 chance of dying, so if you only do 10 of them during your career you'll be FINE), so it's almost at the level where a normal mage can do it just fine. Oh and you got 50% of taking less than 6P damage and being halfway death thanks to damage that cannot be healed in any way except days of resting.
AnyDice: set "explode depth" to 10
output [highest of 0 and (2*(15d [explode d{0,0,0,0,100,101}]) - 11d{0,0,100,0,0,100,100,100,100})]
------------
Now let's take a competent and smart summoner. Assume 19 drain dice and a Body of 9. In other words, K.O. at 13P, dead at 23P. The odds are 1/30 chance at going K.O. and 1/2000 at dying. Oh, and only 1/4 chance of taking at least 6P drain. In other words, for this summoner, if he edges his drain roll vs an edging spirit, he can manage a Force 10 just fine.
A force 13 is at 1/7 K.O. chances, so a summoner who does this often will knock himself out and let lose a really dangerous enemy on his party once every seven times. Oh, and we're at 50% odds of taking at least 6P drain. And he has to cast 3 Support Spells, meaning he'll have to use 3 Spell Slots and reagents (a mere 320 nuyen if he rolls his 4+ hits) to do this without Focus Addiction, and he still has to use up 2 Edge, one for the Summoning and one for the Drain roll.
AnyDice: set "explode depth" to 10
output [highest of 0 and (2*(15d [explode d{0,0,0,0,100,101}]) - 18d{0,0,100,0,0,100,100,100,100})]
set "explode depth" to 10
output [highest of 0 and (2*(20d [explode d{0,0,0,0,100,101}]) - 18d{0,0,100,0,0,100,100,100,100})]
------------
In other words: If we're functioning under the assumption you can Edge both the Summoning and Drain roll, and functioning under the suggested "Spirits above certain Force always use Edge to resist the Summoning" houserule, then even a really clever Summoner will still hurt like hell doing this, at a big price. Meanwhile, the given sample summoner would only do this as Do Or Die, and even a Force 10 would not be something he wants to summon often.
If the problem is high Force Spirits being summoned out of chargen, the solution lies in letting the Spirit use an edge point to explode its counter-Summoning dice. Afterwards it has a fair to good chance at going berserk and attacking the knocked-out arrogant summoner and his party, likely resulting in a TPK.
Now someone cocky enough to summon significantly above his Force MUST consume 1 or 2 Edge Points on it, as well as have a significant risk at the spirit going haywire. If they get way above their head, they are at a significant dying risk. And even the most-prepared Summoner (5 Body, 6 Willpower, 5 Drain Stat, 3 Sustaining Foci Force 1, 3 Increase [Attribute] Spells cast with 320+ nuyen in reagents) will have a hard time swallowing the drain damage and not dare to touch that Force 13 unless he really has to.
So those would be my objective numbers, and a simple GM solution to the matter: "Any Spirit with its Force above your Magic WILL use Edge to resist the Summoning."
