There's three easy ways to kill virtually anything in the game, including all the parts of Vegnagun.
1) Gunner - Gunner - Gunner
Put the cursor to memory, set all three characters to "Trigger Happy" (preferably level 3 for each) prior to the battle, hold the X button and tap R1 until it dies -- assuming you don't die first.
2) Dark Knight - Dark Knight - Alchemist
Have the two Dark Warriors use Darkness, while the Alchemist uses the skill Mega Potion. All you do is sit and hold the X button. It's extremely effective against multiple enemies that the Gunner team couldn't kill effectively.
3) Anything - Anything - Gunner
In two words: Cap Nip. With caution or critical HP, each Trigger Happy shot will do 9,999 damage. Combine that with Trigger Happy level 3 and a good tapping pace, and anything will die in a matter of moments.
There's other ways, too... but I never cared enough to look for them.