Is it even legal to begin with?
I'm going to say no. Yes, 91.15 does allow the dropping of objects:
No pilot in command of a civil aircraft may allow any object to be dropped from that aircraft in flight that creates a hazard to persons or property. However, this section does not prohibit the dropping of any object if reasonable precautions are taken to avoid injury or damage to persons or property.
But 91.13(a) says.
(a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
There is also 91.119(a)-(c)
§ 91.119 Minimum safe altitudes: General.
Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
For an aircraft to be at all involved in a paintball match, it will need to be doing low altitude maneuvers. This will violate one of the above laws. I'd say the most assured is 91.119(a). An engine failure in that situation would be very dangerous to the people on the ground.
I absolutely believe, based on my experience with the FAA investigators, they would not consider this legal.
Assuming it is legal, what is the recommended way to do this? Plane size? Configuration considerations?
My very strong recommendation is that you never, ever do this.
Lastly, is it legal to shoot back at the plane? Or is that just wildly unsafe?
18 U.S.C. § 32 : US Code - Section 32: Destruction of aircraft or aircraft facilities says you can get 20 years for damaging an airplane. I'd also say it's wildly unsafe.
I say this all as a pilot who has dropped his share of stuff from airplanes. Rubber chickens onto a toy house at an airport as part of a competition, toilet paper to tear up with my prop, pumpkins in the marsh and paper bags filled with flour. There is a time and a place for it, and a paintball match isn't it.