Do not upgrade your armor or weapon on a character unless you can upgrade them all at once. There is a 4 piece set bonus that grants more starting rage if all items are upgraded to the same level. Do not confuse the armor's level (phantom, arctic, aurora) with the enchant or enhance levels.
Spread your gems and runes out before combining them to higher levels. One higher level rune or gem has lower stats than the two gems used to make it. Gems can be combined without loss of power up to level 3.
When deciding how to distribute gems: Put defensive gems on your tank. PATK, crit, break-block, break-defense go on your main. MATK goes on your other two. Once you have a lot of gems, you can start giving your non-tank characters some defensive gems, PATK to your tank, and crit to other characters.
When deciding how to distribute runes: Follow the same rules as how to distribute gems. Runes have two stats though, so make sure both are valid for the character. A rune with MATK and block will be half wasted on your tank, who doesn't use the MATK stat.
When upgrading sacred weapons, wait until you can do the whole set at once. The left side gives a damage boost, the right side gives an HP boost.