Traverse the ice field and Bring Moonstones to Mystic are missions in Going Commando taking place in the Tundor Wastes, Grelbin. When Angela Cross invited Ratchet and Clank to meet her on Grelbin, they decided to explore the ice field near where they landed. In doing so, they found the Mystic, who claimed to be capable of lifting a rock blocking the way inside the ice field if he was given nine moonstones found in Grelbin. Ratchet and Clank obliged.
No new weapons are available at the Megacorp vendor, but the enemies fought here are extremely powerful. It is a good idea to purchase the Shield Charger if you have not already, and your weapons of choice will be the Plasma Coil, Bouncer, Hoverbomb Gun and, Minirocket Tube; use this as an opportunity to upgrade any that have not already been upgraded, but keep in mind once upgraded these weapons will be good choices for farming moonstones nonetheless. Your main enemies are the Y.E.T.I. and the arctic leviathans. The Y.E.T.I. are melee enemies that attack in large numbers, while the arctic leviathans are ranged enemies guarding moonstones that are found beneath ice pits and will breathe blasts of ice. Both are very durable enemies and can easily overwhelm you if being careless.
The mission is begun by approaching the Mystic, who you can found by taking the slope down to meet him by the rock at the entrance to the cave. After a short cutscene, head out into the ice field and obtain moonstones either by breaking them from their blue ice shells, or by defeating arctic leviathans. You can obtain a total of fourteen right now, five from defeating arctic leviathans, and nine from the shells.
Once you have nine (or all fourteen), return to the Mystic. He will then offer you a Hypnomatic part in exchange for sixteen moonstones in the mission "Trade moonstones for Hypnomatic Part", though you cannot obtain sixteen yet. Instead, finish the mission "Explore the caves" first.