Official Strategy Site -> https://ffrkstrategy.gamematome.jp
Not always easy to find the informations because searching in comments.

Inofficial Strategy Site -> www.ffrk.kongbakpao.com
I can recommend the site, always having great informations but its from the Japan game version and sometimes with skill suggestions a bit ahead of global game version. Because its from the Japan game you can get a sneak preview of what is coming next, extremely useful if you like to plan ahead events!

Generally I suggest to get the breakdown skills 4*, shellga 4*, protectga 4*... those you will need in most fights, if used both you nearly dont get any damage at all...
For long combats you will need honed abilities. If you dont have good physical weapons try it with mages.
You should spend all Mythril into rare relict draws, that way you will get stronger pretty soon!

For Gygas:
Try to get Sephirot Limit from a roaming warrior to blind his adds, if not you can sleep them and ignore them then...
For the Boss: Poison + Power Breakdown + Protectga and you should not have any problems...

If you like, I can help you with a great roaming warrior (Friend Code): eSoR