In order to find this geocache you will have to download a Wherigo cartridge, and play it on your GPS or any other device which is capable of playing Wherigo cartridges. You will need the file for the cartridge, which you can download at Wherigo.com. If you do not have a GPS capable of playing Wherigo you can install an app for your phone. For Android phones WhereYouGo is the most used, and for Iphone you can use the app called Wherigo.
In order to find the final coordinates you will have to visit all the places before you are able to see where the geocace is hidden. When you have found it you are not very far from a place where you can have a rest and enjoy the very nice view. If you are lucky (or not, depending on how you will see it) you also may spot some of Oslo's fauna, there are some local animals living close to this spot.
The difficulty rating is for the whole tour, which could maybe take some time depending on your chosen mean of transportation, and the terrain rating is for the final location, which should not be too difficult to reach.
Have fun!