Instructions
Fight your way to the sewer on the south side of the map. Neutralize any nearby opponents. If possible, bring an ally with you because you'll need a few seconds of freedom to glitch into the sewer.
Head to the rightmost corner of the alley and face the sewer. Hold the movement joystick (left stick on default settings) toward the sewer and start jumping. You should clip through the wall and into the sewer, if you get stuck, strafe right and left until you are free and start jumping again.
Crouch behind the rocks for cover and shoot any passing opponents. Unfortunately, once you are inside the sewer you can't leave, so your only option is to camp there until you are killed. To leave you'll have to use a grenade on yourself and suffer a small loss in points for your team.