To unlock the Extra Mode, clear chapter 4 in the Story Mode. It will exist on the title screen.
More photon points
Go to the shop and buy a lot of the same weapon that you use. Then, add them all to your palette. Equip all of them with skills, if desired. This trick does not require multiple skill items. This will save a lot of Meseta because you will not need to buy Photon Charges. This trick is also very effective for forces because they will not run out of photons nearly as often.
Unlocking SUV weapons for Extra Mode
Get S-RANK in these missions for them to be available at a shop in Extra mode: You need to be at least Lv. 20 to equip them. Missions in Extra Mode are avaialble after you complete them in Story Mode.
Effect - Code
Blitz Buster - Shrine of Fanatics (level 35 newdaiz)
Grome Buster - Mech Soldier of the Sands (level 40, moatoob)
Hegel Buster - Freight Train Rescue (level 15 train mission)
Raphaal Buster - The Remnants of Endrum (level 35 mission that starts from the subway)