If you're going to sue someone or some organization, you need to state the grounds. What contract has been broken? Also you need to consider what court will hear and rule over the process.
If the UK wants to challenge the EU in court, the best method would be to leave with no deal, refuse to pay the EU any more money, and then wait for the EU to sue.
If the UK wants to challenge the EU in court, the best method would be to leave with no deal, refuse to pay the EU any more money, and then wait for the EU to sue.