As soon as the level starts, you're going to be doing the thief quest clipwarp. This involves setting your clipwarp at the end location before the thief quest, then starting the thief quest and warping back:
While the thief is moving, you're going to be setting your clipwarp position above the level end trigger. Once the thief quest is done, you'll be entering a warehouse and warping back outside, then falling right into the trigger: