Shortcut for opening an app?
You will find several solutions (Keyboard Maestro, Karabiner, Automator Wrapper) or here (LaunchBar/Quicksilver) to accomplish this.

Another Karabiner solution is the Launcher Mode:




Download, install and start Karabiner.
In the menubar open Karabiner -> Preferences
Search for Launcher and enable the preset.
To open Activity Monitor hold OA and release A