Firstly, I had no part in making & do NOT take credit for any of the following information.  Creators are attributed in the actual guides.  Just trying to compile it in one place to help some aspiring Nins that may have not known about it.

The Ninja/Kunoichi discord, full of a bunch of experienced people to answer questions across all platforms: https://discordapp.com/invite/ninjakuno

This is a comprehensive Ninja guide (gear/build theories, skills/skill add-ons, small & large scale tactics, class matchups, etc.): https://docs.google.com/document/d/1U3IoifRGUj88R58Vg3cDQl1ks7lj5ElPquBPUYrzRrA/edit

This is a PS4 Ninja Combo guide (with button layouts to make it easier): https://docs.google.com/document/d/1d1NK5oEF06Gt_x2aqoJTo4BPbqyBiCUZzeV24PMMSu4/edit