It's a fairly long, but mostly hidden process requiring a crouch jump.
The first two jumps are fairly obvious. The ledge 3 is a bit less obvious; it looks like decoration, but it's actually a platform you can land on. It's even less obvious that this ledge actually expands in the area marked 4. Once you are against the 'protection', check the leaderboard for respawn vaves, and crouch jump (tap jump and crouch at the same time). Sap to your heart's desire.
Here's a video demonstration, too.
Actually there's an exploitable oversight right here, as this particular spawn door is not protected and enemies can go through while it's open (but not open it themselves). Be careful and don't be lame!