What should be trade union representatives core central message when entering pay package negotiations?

Your psychological consultancy has been approached by a trade union representative for public sector rail workers. Their members are unhappy with the pay package being offered to them as well as a lack of perceived improvements to working conditions. They are fully committed to engaging in industrial strike action if they do not feel like their demands for an improved pay offer are met and commitments to improving working standards are made. The trade union representatives have asked your consultancy to advise them on how they can make as persuasive an argument as possible to avoid industrial strike action.They have asked you to consider: What should be their core central message when entering negotiations? What peripheral cues could be used? How can they tailor the message to create attitude or behaviour change? Any other suggestions you may have to improve how persuasive they are.