Ogrork the Mighty said:7d6 (or 9d6) x 8 PCs = 56d6 (or 72d6) damage per day
With 4 characters that's still 28d6 (or 36d6).
That's a heck of a lot of damage. And there's not too many BBEGs that can stand up to that kind of hammering, potentially all in one round.
Except that it is hard to target a BBEG with this much damage with Arc of Lightning in a single round for several reasons:
1) Arc of Lightning targets creatures. If you had 4 or more PCs attempt this tactic in the same round, you would have to have plenty of NPC minions around, otherwise, if a minion dies from one of the attacks, he is now an object and not a creature and cannot be used for other Arc of Lightning spells.
2) It has close range. At level 7, that means 40 feet. Any minion used would have to be both within 40 feet of the BBEG and within 40 of the PC casting the spell. And, the BBEG would have to be within 40 feet of each PC casting the spell.
3) Any minion used would have to not have a PC between it and the BBEG, otherwise, the PC would also take damage.
4) Arc of Lightning is a broken spell to begin with since it does not have a saving throw (trading range for a lack of a saving throw is not balance). So, we really should be using Cone of Cold or some other 4th or 5th level spell. The save DC for any other equivalent 4th level spell is DC 16 and for any 5th level spell is DC 17. This is not that high and BBEGs should save against this most of the time.
5) Arc of Lightning does, however, have spell resistance. Many BBEGs at higher level play have spell resistance, so since the caster level is 7, the chances of getting through any BBEG spell resistance is fairly difficult.
6) This assumes that the BBEG does not get an action between the point in time several PCs cast Arc of Lightning and he get his initiative. Sure, the PCs could ready or delay actions so that they all go at the same time, but that could cause issues of its own.
Also, since we are talking a level 4 spell, a 13th level Druid could cast Empowered Arc of Lightning with level 13 against spell resistance and do 19.5 D6. A single Druid here could do 33% (or 67%) of the damage that all 8 (or 4) PCs do. That's 1 action versus 8 (or 4) actions. Just a few PCs casting higher level spells at the BBEG can do more damage than this tactic.
In the large scheme of things, 8 PCs all attacking a BBEG in the same round could probably do just as much or more damage with their normal attacks as with this tactic. This is only 24.5 points of damage per PC. At level 13, most PCs can dish out that kind of damage (or more) in a single round in one way or another when you consider that the spell casters alone are doing twice as much damage as a single minimum caster level Arc of Lightning (more if the spell is Empowered or Maximized).
And as PCs get higher and higher level, they either have to beef up the Spirit Companion, or it gets less worthwhile. Once they get beyond 16th level, they cannot beef it up anymore (and it becomes less useful at high and Epic levels).