I keep struggling with Cisco ASA.
How I can make a certain NAT (RDP, SSL or whatever) and secure it by allowing just one external client with fixed internet IP to make use of this NAT?
This is a start for a configuration example for static NAT. There are significant differences between versions and sub versions of ASA to be sure to reference the right version, and search for the relevant configuration guide .
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/nat_static.html