Similar to how Miradora compiled a bunch of links to guides and sites for outside the forums, here I'm going to compile some guides on this forum for ease of access (except my starter guide series; my opinions on some classes and skills have changed since and I don't feel like remaking them any time soon).

This guide talks about where and how to level in smaller parties if you are struggling to find a sufficient AoE party.
Veigar's Leveling Guide

Following that, GheyMang was kind enough to show you how to struggle through the pains of rebirthing.
GheyMang's Reincarnation/Releveling Guide

Getting decent gear past 50 is important, and this rather long guide covers what you need to know.
The Ultimate Gear Guide

Quite a bit of changes have been made with elemental interactions, so the following thread is a good reference.
Elemental Interaction Changes

Covers everything you need to know about being a tank up to level 50.
Team Mercenary Guide

Mainly the underappreciated parts of Mercenary and late-game tanking (if you can be bothered to be a main tank).
Mercenary: The Advanced Stuff

More a question than a guide but here you will find most recipe and item drops from the Firework Map.
Costume Alchemy?

Various raids.
Garnet Godwin 101
Northern Turtle Raid 101
Southern Vermilion 101

This entire thread is me rambling about reaching 1k Transfusion Tune. I may not have taken the level penalty of level 12 Transfusion Tune (full effect kicks in at level 73) into account. Linking the last post.
The Legendary 1k Fuse

If you see anything on this sub-forum that you'd me to add here, PM me and I'll consider it.