For starters, haters will always be haters. That being said, if you have any information on past costumer complains, I would recommend you develop an FAQ or response protocol for any issues that clients constantly complain about. Most trolls and un happy users have past experiences with the company, or the have heard from other clients that your service and offer. You should have a clear scope of this.
On the other hand, you should asses your campaign and try to asume probable blowback themes. Prepare to responde, or not, most themes are recurring. Users can link your campaign to other conversations, user your hashtag to poke fun at other brands or brand names, and turn your campaign into a meme. If anything, theses are always potential spin-offs you can capitalize with your creative team.
My general recommendation would be to align other areas of the company your are working with, with this, monitor all conversations and if something comes up, you will be the first to know and act on. The worst thing would be to have troll hitting you and now knowing about it... and finding out from your client first.