Once you reach Round 60 or above, acquire a Mangler Cannon Scorestreak and teleport to Mars. Ensure all your teammates are with you, then charge the cannon and shoot it at the teleporter before having everyone go through the teleporter. Once you’re back on the main map, throw decoys one by one next to the portal because the blast will follow whoever shot it when it comes through the portal. The Manglar shot will hit the zombies, taking them out. Repeat this step until you take out enough zombies for the portal to spawn in.