I started writing an answer a few days ago, but didn't finish it because I had to look up some details. It's a very good question, because it's not immediately obvious from the rules how it all fits together. Writing down this answer helped me a lot to get things straight.
Roll Spellcasting + Magic [Force] to cast the spell
If your Spellcasting (or Sorcery, the skill group Spellcasting belongs to) is 9 and your Magic is 5, you roll 9+5 = 14 dice. Roll them. All 5s and 6s are hits. Count your hits. At least one hit means that the spell is basically successful, but for spells targeting people (like Manabolt), the target gets to resist.
[Force] means that the Force of the spell acts as a limit to the number of hits you can get. If you roll more hits than the Force you chose for the spell, the excess does not count. I don't think Mental Limit matters here. Where did you see that that would matter? Force replaces it as the limit for this roll. Note that the Force also determines Drain, and the number of hits you roll determines what kind of drain it is. More on that below.
For now, let's pretend you rolled 4 hits for your Manabolt. It's lower than the Force you chose (6), so they all count. Had you chosen a Force of 3, only 3 hits would count, but let's go with 4 for now.
Resist with Will + Counterspelling
Because of your 4 hits, the target will get 4 points of damage, but first, he has the chance to resist. He rolls his Willpower + counterspelling dice, if any. Suppose his Willpower is 3 and nobody is counterspelling. He rolls 1 success, you've got 3 net hits left. The target crosses off 3 points of damage.
But what if someone is counterspelling? Then you get to add Counterspelling dice to this defense roll. More on that below.
Physical versus Mana spells, Direct vs Indirect spells
Manabolt is a Direct Mana spell. What does that mean exactly? A Direct spell is a combat spell where the magic of the spell affects your target directly; they generally only do damage equal to your hits, and are resisted with Will (for Mana spells) or Body (for Physical spells). Manabolt and Powerbolt are classic examples. An Indirect Spell creates a physical effect that affects your target indirectly. Fireball is a classic example. It does a lot more potential damage (net hits + Force, comparable to a gun with DV equal to Force), but first the target gets to avoid being hit with Reaction + Intuition (like against a gun), and then they can resist the damage with Body + armor (just like with a gun).
So indirect spells are a lot like guns where Force == DV and Spellcasting == your combat skill. Direct spells affect the target directly, and he doesn't get a roll to resist the damage; resisting the damage and the spell are one and the same thing, you get only a single roll: Will (for mana spells) or Body (for physical spells).
But Direct and Indirect spells both benefit from Counterspelling.
Counterspelling
As a free action, a magician can declare she's ready to counter spells targeting her or her buddies for that turn. If she hasn't declared it when the spell comes, she can still do so as an interrupt action that reduces her initiative by 5. The magician can protect a number of people up to her Magic rating. This gives her a dice pool equal to her Counterspelling skill which can be used to add to the defense of one or more attacks.
When the attack spell comes, the magician decides how many dice from that pool are used to defend against that attack. The target of the spell gets to add those spells to his defense roll (Will or Body for direct spells, Reaction + Intuition for indirect spells).
In the case of area spells (Manaball, Powerball, Fireball), the magician spends the dice from the counterspell pool once, but each target adds those dice to his defense.
Back to your Manabolt. You had 4 hits, right? Suppose the target's mage buddy gives him 3 counterspelling dice. The target then rolls Will + Counterspelling = 3 + 3 = 6 dice. Let's say he gets 2 successes. Substract there from your original 4 hits, and he gets 2 damage instead of the 1 he got without Counterspelling. The target crosses off 2 squares of damage.
Resist Drain depends on your tradition
If you're a Hermetic Mage, you resist drain with Logic + Willpower, if you're a Shaman, you use Charisma + Willpower. The drain you need to resist depends on the spell and the force used. Manabolt has a drain of F - 3, so with a Force of 6, you have to resist 3 points of drain. Note that you always have to resist at least 2 points of drain, even if you chose a lower Force.
So roll Charisma + Willpower (since you're a shaman), and substract the number of successes from the drain. Cross off that many boxes of damage. But is it physical or stun damage? That depends on whether your original number of hits was higher than your Magic rating. You rolled 4 hits, and your Magic is 5, so it's stun. But if you'd rolled very well, you could have gotten 6 hits, and it would have been physical. You could prevent that by selecting force equal to your Magic rating. With a Force of 5, there's no way you could have gotten more hits than your Magic rating, so it would always have been stun damage. That is, unless you spent a point of Edge to Push the limit; then the limit disappears, and you get a bunch of extra dice to roll, so that's a great recipe to accidentally get physical drain damage.
So how do you use Force really?
Force is an important tuning dial for your magic. Here's a few things you can do with it:
- Set it equal to Magic to prevent physical drain
- Set it low to make drain easier to resist when you don't need tons of successes
- Set it really high to increase the damage of indirect spells
- Set it really high to increase the radius of your area spells
Of course in those last two cases, you risk a lot of drain. And because of the high limit, that drain could be physical, though it's possible to set a lower limit on your roll by using Reagents (p.316).
