Setting up remote VPN access with aggressive mode disabled...

I have been asked to solve a problem which I'm not very familiar with. I've setup remote VPN access before but not with these options. My company has to be PCI compliant and we have a firewall which we use for some partners to access our internal network.  We were recently told by our auditor that we could not have aggressive mode enabled on the VPN profiles.  But apparently we can't get the VPN connections to work once we turn off aggressive mode. I read somewhere that you can use certificates instead of pre-shared keys and that might solve the problem. But I'm trying to set that up and having problems. Anyone have any insight into how to make this work? (remote IPSec VPN with aggressive mode disabled?)