I'm already in the second year of my PhD studies and my advisor insists on pursuing a very vague track, in order to produce some results that may be publishable. The problem is that the topic itself was extensively studied in the past two decades and there's little maneuver space without overlapping with others' past ideas (it's in applied Computer Science). My adviser seems to be constrained by his grant committee to have a solution for the topic's problem using a certain kind of technique that will be outperformed by other, more specific algorithms right from the start. Although it has other benefits, they're not interesting for this particular topic (since they're not going to be used in any way).
To put it simple, how can you tell your advisor that what they want you to do is like killing flies with TNT and then have the denotation site rebuilt in order to get rid of those flies? (in an elegant way, of course).
I should mention that my advisor has a different area of expertise than the one chosen for my PhD program and often seems not to having the slightest hints on what state of the art means for that particular field and why it is important not to reiterate past methods and algorithms just because they can be applied in a slightly different field from the one they were initially proposed for.