^Trust me on this one.
The game gets ALOT harder when you go through the missions. IIRC the last boss (not Minerva) was a cake walk. The missions become incredibly intense as you progress and individual missions exceed the overall difficulty of the game.
Costly Punch is nessacary if you want to complete the missions, enemies have uber amounts of HP and don't mind swarming you when they get the chance. Plus, it deals the highest possible damage in the game.
Also, Protect Ring is strongly reccomended > Safety Bit, although you have wall, recasting it everytime it fades away is tiresome and can often cost you a battle. Enemies using insta-death are few and far between, but needless to say Death Missle, Death Scythe, Death Shot and Death can all be avoided by a well timed roll - honest!
Alternativly, track down the Genji Shield, it has Auto Barrier and MBarrier, absorbs ALL Elemental Attacks (like Flame Armlet) and blocks all negative status effects, including the effects of Death.
If I recall correctly, my set up was not unlike this:
Curaga (+999 HP)
Quake (+255 Atk)
Darkness (+255 Vit)
Hell Thundraga (+255 Mag)
Mug (+255 Luck)
Costly Punch (+255 Spr)
- Genji Shield
- Genji Armor
- Genji Glove/ Heike Soul (After Minerva)
- Genji Helm/ Divine Slayer (After Minerva)
Thats pretty much 'god' set-up. Stats Maxed, Damage, HP, AP, MP limits broken, no MP/AP cost, absorb elements, auto Wall etc etc.
And even with all that, Minerva still takes around 30 minuites to beat. ^_o Whereas the last boss, iirc took me 3 (the first playthrough with regular non Genji item setup)
See what you're in for ^_^