Simple.
A prop_physics lamp
A Keyframe_rope which is to be parented to the lamp
A move_rope which should be the top part of the rope and connected to the keyframe_rope
A phys_lengthconstraint which is put on the exact same position as the keyframe_rope and should only have one change in the properties, "entity1" which should be changed into the name of the lamp prop
Done with the swingypart!
Now for the light itself.
Make a light_dynamic, set it up as you wish, and then parant it to the lamp prop, and put it directly under the lamp model.
It should not touch the model sides!
And if you wish, put in a point_spotlight to create that foggy light effect.