Heh, this happened to us yesterday. It was the end of a very frustrating day for me. I was doing very well as a tower rook, while Regulus and AI Oak handled the rest of the map. I pushed into the cit, we got catas and eventually giants, and got their cit down to under 10k hp. They must have bought all the upgrades at once, because the cit health jumped to 26000 and kept climbing.
Their rook ported back, and started slamming Giants. He quickly outleveled and outspent me. In no time he had 6000+ hp and a hammer slam that felt like it was doing way more than the 1200 in the description. In retrospect, I probably could have stood up to him if I had orb of defiance, but I wasn't thinking clearly.
They won.