These things can be a real pain, the threads are very fine and corrode easily. I take it you've tried doing it up slightly first? It's also worthwhile wiggling the cable end of the connector if you can.
On the tool front, I find the jam jar opener rubber strap wrenches ideal for these. Available cheaply from your local supermarket.
When you get it apart, remember to put a smear of copper grease on it when reassembling!
Andy