I wouldn't recommend using an extension for the pole saw. Just lifting it up and seeing the heaviness pulling the shaft down to make a slight arc I could easily see how the pole would get damaged from a quick movement of lowering or raising the pole from momentum. Either hiring a tree climber, using a manual pole saw with or without a ladder or choosing to drastically make shorter cuts from the base or totally eliminate a tree I'd say would have to be considered if the basic 12 foot Echo pole saw cannot handle the job.
