This is available under the "Extra Missions" menu at the title screen. You need to obtain close to all of the Ex Missions (I had 93% collected when this became available) in order to unlock this one.
Zoradius is located under the Vic Viper missions (and is the only mission for it
). The best way I can describe this mission is Starfox for the SNES - this mission is Vic Viper, in jet form, shooting enemies down a tunnel.
The key for victory here is to get every, and all, of your weapons ASAP. As you are flying through this tunnel you will see red orb-y looking things. This allow upgrades to be obtained. There are blue orb-y looking things as well - these will instantly destroy everything on the screen.
In terms of upgrades, get yourself the following fast:
- Option (2)
- Ripple
- Missile
- Laser
The laser replaces your standard square button attacks. The Ripple is activated by pressing the circle button, and missiles are activated via the X button. Some may consider it "cheating", but I just taped down all three buttons and raged through the level.
The Speed upgrades you can grab whenever, but honestly I found that too many had me flying all over the place. I found 3 - 4 to be the best maneuverability wise.
The "?" you should build up and save for the last enemy: a giant vehicle (looks like the train in the main game) that has a wicked arsenal on it. It was a pain to beat UNTIL I obtained the "?" upgrade. It is basically a shield that absorbs X amount of damage.
Question, corrections and neg vote justifications in the comments below.