Hello,
We are working with Payment gateways since 1996, and actually working too on PCI compliance.
This task is very dependant on your system, I can tell you how we are doing to success on scanning.
I can also access your system and do all task to secure it, but again there some dependency on what system are you using.
We move to use our system on separate lan and no any other service opened, and of course a very strict firewall rules, early intrusion attempt detection, auto blocking some possible attacks and more.
In our case we own the full network management, including the BGP routers, switches, etc that for sure is not your case if you are hosted on some provider, but that is not any inconvenience.
First of all I would like to know what OS, version and what other services are you running in the same host.
Best Regards