Load Mission 5: Siwa Oasis, equip the Panzerschreck RPG, and select any difficulty except Authentic. The Authentic difficulty does not feature in-game saves, which will prevent you from using this exploit. Skip the cutscene so the personnel truck is in range of your RPG.
When the mission begins, run into the field, pause the game, and select the “Save Game” option. Then, destroy the personnel truck, pause the game, and select the “Load Last Save” option. Keep destroying the personnel truck to get 100 XP approximately every four seconds. Repeat this process as many times as desired.