This is why there are decoy flares.
Anyway, WWII-style antiair guns would easily down a F-35 in a CAS role unless it is so high that its ability to handle that is compromised. Things that shoot bullets are the real threat to CAS aircraft. Also, the F-35 has so few shots that it can only fire a fraction of the bullets that the A-10 could fire. Combine that with reduced accuracy from having to fly higher up and it is almost useless in a CAS role.
That being said, it is lousy in all of its roles. Multirole aircraft that are capable of fewer roles are far better than the F-35 in just about every role imaginable. The F-22 is an excellent example of this. :/