He used a dangerous weapon on you. You can make a citizen's arrest, in all probability. However, citizen's arrest isn't a euphemism for beat the crap out of. Make the arrest, and dial 911 stating to the dispatcher that you've made a citizen's arrest for assault with a weapon. If the guy flees, follow; You might get in trouble for trying to restrain him from fleeing, though this varies by state (in some states you legally COULD restrain him).
However, if his response to the arrest is to assault you again, then you are legally able to use force in self-defense. Like citizen's arrest, self-defense is not a euphemism for beat the crap out of. Force must be proportionate. Bringing a knife to a fistfight, for example, is a good way to end up an imprisoned felon. You can defend yourself until he stops attacking; If you keep "defending" after he stops fighting, you're the one going to jail.
For example, I live in Washington state. A citizen's arrest can be made here for a felony or a misdemeanor, but only if the misdemeanor breaches the peace. A citizen has no investigative powers however, so a citizen's arrest must be immediate upon witnessing the crime, and the citizen making the arrest must be a direct witness. Waiting to arrest him later or arresting him because your friend told you about the actual crime are reserved for police, exclusively.
Assault and battery are both breaches of the peace at the misdemeanor level here. Some states classify a paintball gun as a real weapon, so in those states, him shooting one at you may be a felony. But Washington doesn't, so it's a misdemeanor.
The proper procedure for the arrest is to approach him and inform him he is under citizen's arrest. Immediately (or possibly while you walk up to him) dial 911 on your cellphone, or call 911 as soon after the arrest as possible if you don't have a cellphone. Inform the dispatcher you've made a citizen's arrest, what the arrest was for, your exact location and request a uniformed officer. Stay on the line with the dispatcher until police arrive. Perhaps even use the speakerphone function if you have it, so the phone can capture what the arrested individual is doing as well. Video cameras are useful here too, if your phone can take video while making calls.
If the arrested individual resists, Washington law does not distinguish between citizen's arrest and police arrest; Resisting either is a crime. However, restraint must be proportionate. Wrestling is probably okay, punching isn't okay, pro wrestling definitely isn't. However, this changes if the arrested individual attacks you as part of that resistance; At that point, your right to self-defense kicks in, though again, proportionate force is required.
It's worth noting that nothing in Washington state law renders a uniformed police officer immune to citizen's arrest. There is a law on the books that makes forcibly disarming a police officer a crime, but exceptions exist to that law for two circumstances: A judge can order an officer disarmed (even if it's a citizen who actually does the disarming), and if an officer under citizen's arrest attempts to use his weapons to resist or escape arrest, then he can legally be disarmed.