I wouldn't necessitate a lower force elemental attacking a non-optimal target.

Instead make their tactics simple, attack the closest, the largest, that kind of thing. The smarter the spirit the more complex tactics they may use, taking into account magical talent, size and types of weapons, cover, that kind of thing.

A really low force elemental may simply go straight for the closest opponent. A slighly higher force one may actually try to use cover in their approach. A higher force one may try to geek the mage first. That kind of thing.

And indeed, keep track of how a magician treats their spirits, if they treat them badly, spirits might start deliberately interpreting the orders dumbly, or attempt to be overcautious in combat situations.

I'm happy for a PC to say "Attack them" for one service. If the PC starts trying to exert fine control such as telling the spirit to use a particular power to accomplish something, there goes another service. Extension of the conjuror's will or not (it's never laid down for certain, either way), the GM runs the spirits and the player hopes for the best. A force 3 has average human intelligence and will use mediocre, but not crap tactics. A high force (5 or 6) is likely to act very intelligently in fulfilling its orders, including judicious use of its powers where useful. I don't want spirit summoning to be an opportunity for the GM to punish the player. They do pay for these spirits usually, after all.

I'm also willing to use group or individual interchangeably. You can say "use your power" on this group or on this enemy (if it's a power that can be used on more than one individual). You can't mix and match once you've done that though, And if new enemies arrive then I'd probably say they aren't covered by the original service. It depends on the circumstances and the spirit and, as other's have said, just how the spirit feels. The most important thing to make clear to the players is that it's not a game of carefully phrasing the service request. These aren't genies from the Arabian Nights. If a player requests something that is more than one service, then they don't get it. Likewise, spirits are not usually plotting to pervert the players wishes due to lax phrasing. The GM decides what constitutes a service - it's not that a spirit is summoned and contracted to fulfill whatever the conjuror says. A mage being powerful enough to get complex tasks fulfilled is represented in the rules by achieving multiple services or binding.

I'll also play the spirits with some personality where appropriate, especially if a player dares summon particularly high forces. A powerful Beast spirit may enjoy combat and gleefully pursue fleeing security guards over the course of the next hour, playing with them like a hunting pack. A great form Fire Elemental may get caried away and start the whole warehouse burning to the horror of the conjuring mage. Fun is the name of the game and if a player goes to the effort / expense of getting himself a Force 6, he should get some satisfaction out of it.

Doesn't mean I can't hit the players with something powerful to compensate, though.

That's a lot of services burned up. When you say 1 attack, do you mean "Attack that guy until he's dead" or "Attack that guy in this init pass"?

Spirits are tough. And unless you have magic or decent CHA and WIL, you're as good as dead.

Spells are tough. And unless you have magic or decent BOD and WIL, you're as good as dead.

Assault Rifles are tough. And unless you have cover or decent armor and BOD, you're as good as dead.

I'm not sure that a spirit is that much tougher than the equivalent. A Force 6 spirit is pretty nasty. But summoning a Force 6 spirit is no cakewalk, and getting more than a couple services is rare, until you're a 3rd rank initiate. At that point, the Sammy is dual wielding panthers anyway.

My take ( I think it's not an uncommon one, however):

Spirits can use Edge to resist summoning and binding, along with using it in attacks, etc. But this doesn't normally happen in any of these cases. In the cases of summoning and binding, I would have a spirit use edge if the conjuror had a really bad history with spirits. For example, often using bound spirits to do long term spell sustaining, irretrievably burning away their force. To begin with spirits would just use Edge to resist sometimes. Edge can make a big difference to success, especially with binding. I might also have spirits use edge to resist if they had another particular issue with the conjuror. For example, conjuring a forest spirit when your group had indiscriminately burnt or damaged the forest, were working for a logging company, etc. That sort of thing. The rule would be the higher the spirit's force, the more likely it might express its own agenda. And of course, they're precisely the ones you least want to do that.

I would never, under any circumstances, allow a mage to order a spirit to use Edge. I see edge as a meta-game concept and I don't see the mage saying "try harder" to his spirit influencing it much. In the previous examples that I mentioned, however, I can see spirits sometimes using edge when they want to. For example, if you have a Force 7 Earth Elemental stomping about the place, quite frankly it might start enjoying knocking the building down. And the other case would be using Edge to protect itself. Again this would only apply if the spirit were pissed off and really wanted to win. Only the higher force spirits will have such an individual nature that they'll get this emotionally involved.

If the same spirit is regularly summoned or has been re-bound continually, then probably you can let it develop more personality and spend edge more often (either to resist binding or to help its master depending on the relationship).

Edge can really throw a surprise to the conjuring player, either a nice one or a nasty one. Bear in mind that Force 5+ spirits have a lot of edge.

That seems to be the gist of it. The new Street Magic book actually attempts to address this odd physical property of manifested spirits. It explains that the physical bodies of all manifested spirits are composed of the same previously unknown protoplasmic element regardless of their actual appearance. Although the material in each class of spirit has a property unique within that class of spirit that will react with the oppositional element, fire spirit with water.

In 3rd Ed, manifested spirits only exist as a projection to the minds of all nearby living creatures They don't have a physical form. Materialised spirits though do have a physical form, immunity to normal weapons, +10 initiative, etc etc.

I think it would be reasonable that if the mage knows there are 5 guards and says "kill the guards" the elemental will kill those 5 guards, regardless of whether the mage can still actually see them. As long as the mage just has to turn a corner or such that's good enough. If he's a full mage, he could even project (after calling the elemental) and safely point out the target that way.

If there was another guard the mage didn't see, even if the elemental spots the extra guard I doubt it would go out of its way to attack that one. I base this on the fact that the mage can issue commands telepathically, so he could relay to the elemental that he is aware of 5 guards.

As far as physics is concerned, while spirits have privileges in the physical world (like the +10 INIT) otherwise they are subject to gravity except in the case of air elementals. You never see earth, water or fire suspended in the air... and air is just air.

One service to materialize and one to attack is fine if that's what you're leaning to.

I would not let the PCs run an elemental. Remind them that its not a <s>puppet with strings</s> power-suit of sorts, its not a happy camper, and its going to do things its own way--albeit sincerely.

EDIT: On second thought, elementals are rather like puppets.