I've never faced a 'chuck wielding momo as an opponent, but I suppose I could imagine a few things to consider.
Leave if at all possible.
Escalation is always bad idea (he has chucks, you get a 6 shooter, he breaks out shotgun, you lauch a nuke, ad nauseum). However, if you're wearing a belt you might be able to use that in some way to give you some range and to interfere witht he swinging chucks enough to get inside. I doubt you'll do any damage, but that isn't the point; it would be a means to an end.
Pick up a handful of dirt or gravel if possible and keep in your hand. It may be useful in your immediate future. I shouldn't need to explain that.
If you cannot get away, and must face the opponent unarmed, then you must work to minimize the advantage he has; I see that as primarily range; he can hit you but you can't hit him. If you can move quickly enough and use angling to your advantage, you should be able to get inside; if you can, grappling would be the next thing to move into. If he is stupid enough to still have his hands on the flails, you now have the advantage. (one caveat; if he is an exceptional fighter, he will likely improvise and use the sticks in a sort of escrima fashion to reduce the range and fight very close in, and my previous comment is void).
If you do take a blow, try to spread the impact out; in other words, don't let the entire force of the blow focus on one point like the edge of your forearm or the tip of your noggin'. Use physics to your advantage; also be aware of large muscle groups that might be disabled temporarily by a hard impact like your lower back, thights, deltoids, etc.
Also, try to "slip" any impact, in boxing fashion.
I've always thought 'Chucks were flashy and scary looking. I've never actually seen someone spar or be a real fight with them. I would imagine two skilled users going toe to toe would be a short, fast, and furious fight.
That's my $.02.