Stand the raiders you were already making outside their base. Do not engage, but make it very clear that if they move out you're gonna deck 'em. This should slow down their push considerably as they build defenses to keep their base alive. Use this time to rush an answer.
My personal favourite is sharp-shooter. Turn off cloak until you need it. SS will also answer pretty much anything else from the factory, and by the time they know you have it you already sniped their felon. It is slow to build however, so you've got to make sure you can last long enough for it to come out.
If the felon is already on its way by the time you scout it, it's less likely you'll have time for sharp-shooter.
If the felon is alone then you should be able to just build up forces until it's almost at your base then kill it. Against an Unsupported felon, a decisive strike will make cost at this point. Don't piss about, you're all-in on killing it. You've then got a superior economy and reclaim -> strong advantage.
If it is supported, you should have definitely known it was coming, but if
somehow you find yourself in this position...
Building solars around the outside of your base will make it difficult for them to assault you without wasting all of their energy. Presumably you've got raiders? Well keep them alive and wait. Build up high hp/cost units like dirtbag or other assaults. The cheaper the better. Send them in just as they start building their towers, and be ready with your raiders. Once shield is drained on felon, send in raiders and gg.
Afaik felon rush is not a viable strategy against players that know what they're doing.
