[Wdll]

Well, you can either try to destroy the air refences first, or try to work around them.
SEAD aircraft are very important in both cases. They can, usualy, avoid getting hit by sams much. Use this to your advantage, to locate and destroy the sams or to organise an attack against the other targets you want to you use your planes, by attacking first with the SEAD planes, draw most of the enemy fire and then letting your strike aircraft hit their original targets without much threat.
Now, if the enemy has bought a stupid amount of air defences, then just deal with it by not buying any aircraft and basicaly making all his purchase points towards air defence, a waste of money.