How do you get people to try a new food product that is up against a big player?
You need an aggressive stance, probably around the concept of being the people's champion. It has to come from a product truth that taps into the way people view the product and category. So for a product like butter, for instance, this could be a taste or authenticity angle. In line with this, you could also ignore the other players in the category and focus on the market leader, giving consumers the impression you are larger and more popular than you really are. I would suggest thinking along the lines of a "taste challenge" - but remember, the product has to live up to expectations. That's the risk you are taking.
Have you seen examples of large incumbents trying to undermine challenger brands?
One example was the British Airways campaign it waged against Virgin. This involved trying to solicit premium customers from Virgin by offering them free upgrades at the airport. It attracted a lot of bad publicity for British Airways.
Other examples are where the market leader is trying to behave like a challenger itself, such as Dove's Campaign For Real Beauty. It's a great pre-emptive strategy. See the latest Dove Sketches campaign here so you can see how it's working.