There's an optional rule that for direct combat spells (stun*, power*, and mana* spells, but not others), you add net hits to drain. This rule isn't used in missions because it's optional; if it is used, it changes tactics a little but not character builds - I'll get back to this.
It's easy to summon spirits. The problem is that they have very spiky drain, so you want to be reasonably cautious about summoning big ones. The rocker rolls 1 (summoning) + 5 (magic) + 4 (focus) + 2 (mentor) = 12d6 to summon a force 5 fire spirit, and the spirit rolls only 5d6 to resist, so it will almost always work on get a bit more than 2 services on average - and on average only 3.33 drain. The trick is that the drain is very spiky because it's twice the spirit's hits, so it might well get 3 hits and you take 6S. Thus, it's best to summon early, and sleep off the stun, when you can.
To get better at summoning, the best path is to get more drain resist - the limiting factor is really the drain you take, not the summoning test.
I have no idea how much a guitar is on its own, but I expect that's a reasonable choice for what one of the Magical Rocker's foci is.
On the "hits add to drain for direct combat spells" rule, what you want to do if that's used is always overcast your direct combat spells so that the base damage is enough to take down your target, then drop all net hits (which you are explicitly allowed to do). So if that's used, rather than f7 stunbolt, you cast f10 stunbolt and drop all (or maybe all but one, if you think your target has 5+ willpower) net hits. This is because each extra 2 force is 2 DV and 1 drain, but each 2 net hits for the 2 DV would be 2 drain, so you always overcast direct combat spells at the maximum. This is why that rule is dumb, but some people like it.