They definitely exist- I found them on a few sites, e.g.
https://soundcloud.com/opposites-attract-band
However I am not sure mild and bitter are opposites- the setter would have gone for something like bitter and sweet, positive and negative, black and white.
I did wonder whether the mild and bitter might be references to flavours or tastes, rather than ale/beer. I can't think of a band/singer though.
As for large area I have thought about park, estate, field city, town, country, Asia etc but come up with nothing that fits with the rest of the clue.
Going back to beer I did think of coupling BEER with LEA (area of grassy land) or Leigh (town) to give what sounds like Billie (name of Billie Piper on her early hits)