If something has particularly irritating DR you just break out the energy damage splashes (by that level, it's trivial to teleport to a red sea and get enough liquied salt to depopulate the elemental plane of slug). Assuming, of course, you don't just hunter's eye for enough sneak attack to blow through the DR. Or buy some colossal arrows of the appropriate funky materials to ignore DR. Or exploit the endless force javelins from the manticore set item. Or iaijutsu up. Or hit them with metamagiced fire shuriken. Or any number of other things you can do.
True, DR can be irritating, but that's the tradeoff for getting a significantly better ROI when things like knowledge devotion or precision damage get added for each individual attack, rather than just once.
Spells with a single target like you were suggesting have to deal with shit like mirror images and concealment. Telekinesis can strip those off in the first two salvos, leaving the remaining volley able to fire, as well as making common immediate action counters to single attacks (like abrupt jaunt or shadow cloaks) significantly less useful.