Fight your way up to the building to the left from your beginning viewpoint. When at the back of the building with the stairs, go to the top or just stay on the ground, You should see a large wall with a nice designed bar gate. Press Y while aiming in the middle area where there is snow. Once at the top on this ledge aim at the tip of the gate. Make your way over the gate to skip the fight with the large mech below the right building.