Depends on the chapter you're in and if you have the growth egg or not. And as far as gil goes, incentive chips ator, if you can kill the giant turtles, they drop platinum ingots, and gil is what you need to upgrade your weapons so you can buy ultra compact reactors and stuff, here, this is what people buy and use to upgrade their weapon all the way, in the exact order, and the most cost effective way....
Spoiler: edenhall, from the soldiers
Once you have one Trapezohedron, you can turn that one into three.
To do this, pick up a Mistilteinn, one of Vanille's weapons, buyable from
Plautus's Workshop for 11,000 gil. Along with this, you will need the
following materials:
159 Sturdy Bone \
89 Superconductor \ Total gil:
2 Particle Accelerator / 1,507,400 gil
28 Ultracompact Reactor /
1 Scarletite
1 Trapezohedron
Now, use the following items to upgrade the Mistilteinn in this exact
order (for best cost effectiveness)
36 x Sturdy Bone
50 x Superconductor
1 x Scarletite
36 x Sturdy Bone
2 x Ultracompact Reactor
15 x Sturdy Bone
27 x Superconductor
1 x Trapezohedron
36 x Sturdy Bone
26 x Ultracompact Reactor
36 x Sturdy Bone
2 x Particle Accelerator
12 x Superconductor
The weapon should now be a maxed out Nirvana. Once you dismantle this, you
will receive:
71 x Monstrous Fang
60 x Entrancing Tail
36 x Moonblossom Seed
10 x Tear of Woe
3 x Trapezohedron
The items he used to upgrade that weapon, are what you should use on your weapons, they give best exp and you will save a lot of money.
If you can't buy any of that, stick with turbojet or perfect conductors. =]
A good place to CP farm is Mah'habara
Spoiler: When you ride the ball thing back from Fang and Vanille's old home place, forget what it's called.
There should be rust puddings and bombs, it leads to a dead end where A mission is... I really forget what mission, you fight a tyrant before you can get to the mission, if that helps, with a growth egg I could gather 150k cp in 6 minutes.
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