It did feel cheap, but this Pain didn't have that much offensive capability besides his Lie Detector. Having him fight one-on-one would have also put his effectiveness down a few notches.
Kakashi would have taken this guy out in an instant if all he had to do was avoid a grab, while Konohamaru wouldn't have gotten anywhere near figuring out Almighty Push's cooldown.
Konohamaru using Naruto's clone idea to pull off a Rasengan to me is actually the only believable way to do it. It IS a rank A (or AA?) technique where containment was the key. Having him do it without clones would mean his chakra control was beyond Naruto's.
What "is" a tad unbelievable was that he was able to call in 2 Shadow clones AND have enough chakra to pull off a Rasengan. So far, Naruto and the Third have been the only ones to summon 2 or more shadow clones at once.