Access big gun control panel is a mission in Up Your Arsenal, taking place in the Nefarious BFG, Koros. Following Clank's suggestion, Ratchet fought through Dr. Nefarious' forces to get inside the command center in order to use the large cannon that could destroy the Biobliterator. Once inside, the two fought a garrison defending the center in order to give Clank access to the cannon and use it to destroy the Biobliterator.
When you begin the mission and head inside, you will be greeted by a yellow troop transport, which will fire bullets and release a few soldier bots before leaving. Try to take it out with the Plasma Coil or Annihilator before it leaves, while the soldier bots can be destroyed with the Plasma Coil, Bouncer, or Rift Inducer. After you destroy them, head to the right up a small ledge, where a group of soldier bots are waiting on the ground along with some on a bridge. This area is a great opportunity to level up the Holoshield Glove by having it absorb fire (and at higher levels, it can destroy nearby enemies or restore your health).
After this, head right and continue to destroy the soldier bots that appear in each area, until you reach a ramp leading upwards. At the top of this is a corridor with mega bots and several soldier bots warping in, but also an inferno crate on the left edge. Use the inferno crate to dispatch as many enemies as possible, and destroy the rest after it expires, then head right to a bridge. Another troop transport will appear and fire at you, which once again can be destroyed with the Plasma Coil. Head inside the control panel to complete the mission.