I was just gone for a week, so perhaps this is hypocritical, but lets see if we can get some momentum to push through this. If you guys can't think of anything to do, retreat and post a single line describing it. That's all I need to keep us moving forward.
I'll give everyone a day or two to post, and then I'll DMPC everyone to move back. Then at least the invisible person will be forced to follow you if they want to continue casting spells on you.
First I'd like to address the arrows vs invisibility issue. I did some poking about, and there are three reasons why arrows don't beat invisibility. Before reading, keep in mind physics and logic have no place in a PFRPG rules disagreement, merely rules as written. The first and most important reason is:
...ammunition that hits its target is destroyed...
Therefore, there is nothing to see or visually track. Once the arrows hit the creature, they do damage and then cease to exist.
The second reason is:
HP is an inherently abstract system. The arrows could just as easily have missed them entirely, but to dodge them used up a chunk of his HP awesome juice. (Same reason the Stormtroopers never hit Luke/Leia/Han, they were hitting their AC sometimes, but they were PCs with a good chunk of HP to chew through before actually hitting them.) Because getting hit directly with an arrow from a war bow anywhere but a limb means you're pretty much dead (sooner or later) without modern medicine or magical healing.
The third and weakest reason is:
If an invisible character picks up a visible object, the object remains visible. An invisible creature can pick up a small visible item and hide it on his person (tucked in a pocket or behind a cloak) and render it effectively invisible. One could coat an invisible object with flour to at least keep track of its position (until the flour falls off or blows away).
As a DM I would rule that an arrow sticking into someone who is flying around and moving constantly, and wearing heavy robes would be virtually impossible to see on a person 10+ meters from you, even if they were visible, much less if anytime part of their body or clothing covered it it became invisible.
Finally, I would say as a general maxim, that PFRPG is a permissive system. When played using RAW, you can only do what the rules you can do, and nothing else. Since it doesn't say anywhere that arrows overcome invisibility, they don't.
I understand a long combat with no way to overcome invisibility is frustrating, but I hope you can understand my reasoning. You may reply to this post of course, but this is my ruling and I would appreciate graceful acceptance. Cheers~