The fact that such an obvious and simple attack has never happened suggests it can't happen. Shouldn't you realize that?
Well, take care there lots of things are busted without ever being noticed.
Then it is even easier to perform this attack, in theory.
All you would have to do is create a whole bunch of low-difficulty blocks with nearly the same timestamp, then after the "difficulty adjustment" in your branch of the blockchain would result in a super large difficulty. Solve that one block and the blockchain is broken.
This from the guy who was going around claiming to sell a bogus magical ECDSA cracker. I guess the deadline has passed for my challenge, no keys broken? So sad for you.
In any case, no this isn't actually interesting either because you have to do as much work as the whole network to get ahead of it in terms of expectation. So you might as well say "you could go mine as much as the network until you get ahead of it" something you can't do without more computing power than it (much more, in the case that you start far behind it) since the expected required computing power would be equal. The only change is the variance. (and indeed, you can construct some kind of not very interesting very low probability example out of the difference in variance, but like your fraudulent ECDSA cracker, its not very interesting in practice)
(And since you don't seem to understand any of the technical details about the system at all I guess I also need to point out that the difficulty can only increase by a factor of four per retarget, though thats not really necessary for what what you're talking about to not bay a concern, though it does frustrate an attempt at a lucky roll).