Defend the planet is a mission in Ratchet & Clank, taking place in the Tobruk Crater on planet Novalis. After Ratchet and Clank crash-landed in the Tobruk Crater, they found the planet under attack by the blarg, due to Mayor Agnogg Buckwash' nephew Skidd McMarx accessing illegal hydroharvester schematics. The two then sought to defend the planet from the attack.
When you first arrive, Ratchet will point out the raritanium near the edge of the cliff ahead. Break the raritanium free to obtain it, and you can use it to purchase specific weapon upgrades. When you use the vendor, press to enter the Upgrades menu. This will lead to a tutorial in which you must upgrade the "Tutorializer". As instructed, select it and use move down to an available icon and select it. Use the button to select adjacent icons and select the final icon. This will unlock them all and reveal the mystery upgrade in the center between them. After this, you return to the upgrade screen. However, first exit this screen with . You will find two weapons available: the Pyrocitor for 2,000 bolts, and the Proton Drum for 4,000 bolts. Both are useful on this planet, though the Pyrocitor is more affordable. Purchase the Pyrocitor, then return to the Raritanium upgrades screen. Upgrading any of these weapons is essential, though the Combuster's rate of fire, the Fusion Grenade's area of effect, or the Pyrocitor's stream width are the best upgrades at this stage. You should also refill your ammo.
(Note: Those with the pre-order bonus will be able to purchase the Bouncer for 100 bolts. This is the most powerful weapon in the game at this stage, and should absolutely be purchased now.)
It is likely during the next mission that one of your weapons may also upgrade automatically from level 1 to 2. Each weapon upgrades with use, up to a maximum of level 5, at which point a weapon reaches its final version. Using one weapon at a time is an effective method for upgrading each one in combat. However, if you do this, ensure that you still swap out the weapon for another when not strategically viable, specifically if it is weak against a certain set of enemies in an area you cannot complete.
Take the elevator down to the area below and head left. Here, birdbots and peckbots can be found hovering and flying in the air, but they will descend for an attack as you approach them. Birdbots fire ranged blasts, while peckbots purely attack in melee range. Dispatch of peckbots with any weapon (though the Pyrocitor is quickest, or the Fusion Grenade with some area of effect raritanium upgrades purchased) before defeating the birdbots from a range (with either the Fusion Grenade or the Combuster). Defeat enemies within this area, then head up the stairs on the far left, going into the structure connecting to the bridge.
As you cross the bridge, a blarg saucer fly overhead and destroy parts of it, requiring you to avoid falling through. Defeat the birdbots on the other side and cross the other side, and Mayor Buckwash's crashed ship will be revealed across a path. Cross the path, defeating birdbots and peckbots along the way, and you will encounter a group of birdbots and blarg troopers surrounding Buckwash's ship. Defeat these, and Mayor Buckwash will exit his ship, providing an infobot with coordinates to Aridia. This completes the mission. Upon pressing the green button in front of the door, you can return to the elevator at the start of this path and return to the ship.
Returning to your ship will also complete "Find someone to help you repair your ship", assuming the Plumber has already been rescued. If not, take the elevator up to your ship, turn left into the tunnel leading to the waterworks, and complete that mission in order to be able to travel to other locations.