So as I understand it, there is 0.0085 BTC sitting in 3FEmdbfBQUhxppU4yPNFM9phx295SSskib that you are trying to get.
You will need to create the transaction, sign it and then get the "mediator" to sign it as well...
Use the "New transaction" page at Coinb.in (
https://coinb.in/#newTransaction)... Paste in the reedemscript... It will find the inputs, set up the outputs (leaving enough for a decent fee)... And create the unsigned transaction...
Then use the verify page to verify the transaction has been created ok... Then use the sign page to sign the transaction.
After that, you'll need to send the "partially" signed transaction to the mediator and get them to sign it and either send it back so you can broadcast, or they can broadcast it on your behalf