How difficult is it to change out shocks / struts for a pretty good back yard mechanic? can it be done w/o a spring compressor?

 

It really depends on where the car has been living. If it is a southern car or southwestern car the shocks should be easy. The struts will require a spring compressor and they are difficult to get off as the suspension will bottom out before the strut comes out of the steering knuckle. If as you say you mechanic knows what he's doing it shouldn't be a problem.

If it is a rust belt car not much changes with the struts except the addition of rusty bolts. the shocks on the other hand will rip the lower bushings apart and spread the mounting brackets on the hub carrier and snap off the welded nut. Then you will have to use a torch to cut the remining shock lower mountings off without hurting yourself or the car. if a torch is available and the mechanic knows how to use it, again not a difficult job just messy. If someone in the shop cuts shocks off they are usually beaten down for the next few days just because of the smoke and smell, that rubber is nasty when burning. Also be careful as the upper mountings are welded studs and if you break one off the floor of the car, oops! No more shock on that side.