Fly to Oslo Lufthavn Gardemoen, rent a car or take the bus to Trysil. It is only 160km from Gardemoen to Trysil.
Bus communications between Oslo Lufthavn Gardemoen and Trysil are good. Trysilekspressen has several departures every day and it only takes about 3 hours.
Press here to find Trysilekspressens time table.