So, what to do. If it is too late to climb out of the icing you better get down quickly. Since there is a lot of drag don't be afraid to add all the power you've got to keep from stalling. Your stall warning indicator should be heated to help you in this. Remember stall speeds are not what they were with a clean airfoil. If you are on approach, declare an emergency and expedite descent. Again, don't change the aerodynamics by adding flaps or dropping the gear early. Wait as long as possible to do the latter.
Using the the autopilot may be dicey as it will try to maintain heading , altitude and or glidesope irrespective of whats out there on the empennage and may put you in a stall. Therefore it would seem best not to engage it.
The moral then is to avoid ice, but if you are caught remember to keep the airspeed above stall anyway you can. Flying with ice is like pushing a baby carriage full of nitroglycerine. Better do it very carefully.