ATK is the most effective stat in the game.

Warriors with full Ascended armor (720k Shard cost) will have far and away the best stats in the game.

Assassins have a high natural ATK and a set bonus to ATK, making them a strong second.

Sorceresses have a really easy time building INT, making skill crafting much easier. This can make up for a lower overall ATK score.

Tanks and Healers are at an extreme disadvantage when it comes to combat. They gain survivability in exchange for significantly lower damage potential.

Use Death Invitations for +15, farm Freedom City Strongholds to build two sets of Epic gear.
Set 1: Main set with ATK as secondary stat for combat
Set 2: Backup set with INT as secondary stat for crafting

Upgrade the Grinder to convert all unused gear to Dark Cubes.

LoM is in very early development, so all of this information is subject to change. I'm also not completely sure I have everything correct, and comments, corrections, and feedback are all welcome.

There are six character stats in the game:

Attack (ATK), which affects all damage done with every skill, including melee, ranged, and spells.

Defense (DEF), which gives you damage reduction.

Critical Chance (CRIT), which determines how likely you are to deal bonus damage with each attack.

Health Points (HP), when this hits zero you die.

Magic Points (MP), when this hits zero spells no worky.

Intelligence (INT), which (I believe) only affects permanent upgrades of skills after skill level 10.

Gear will drop as either Tier 1 or Tier 2, and regardless of tier, as Common, Rare, or Epic quality.

Every gear piece has a primary bonus that affects one stat, and most will have a bonus that affects a secondary stat. ATK is the most effective secondary stat in combat, and INT is important for skill crafting. I recommend maintaining a set of each.

Tiers can be upgraded up to Tier 3 by spending Dark Cubes at the Forge, but quality cannot be improved.

Gear primary stats:

Helmet (DEF)
Armor (HP)
Gloves (ATK)
Greaves (CRIT)
Ring (MP)
Pendant (INT)

The values for these stats are based on the tier and quality of each item, and improve at the Forge at a predictable rate. This means that there is no random roll involved in determining the value of the primary stat.

When gear drops with a secondary stat, that stat and its value are randomly selected. Higher quality gear will roll a higher value for a secondary stat. The secondary stat is NOT increased at the Forge, even when improving to the next tier.

Stat caps on Epic gear:
ATK +40
INT +10 [citation needed].

I'm unsure of whether Tier 1 Epic drops roll from the same table at Tier 2 Epic drops.

Each class has a unique distribution of stats independent of their gear. These values are set at character creation; oddly, they don't change as your Awakening Level increases. Ranked from highest to lowest by ATK:

1. Assassin 154
2. Sorceress 144
3. Warrior 134
4. Healer 124
5. Tank 109

The gap between Assassins and Tanks isn't massive, but it's worth considering especially in light of the way set bonuses work.

Every class gets a set bonus for having all six gear items of the same tier, but not all set bonuses are created equal. Set bonuses ignore gear quality.

Warriors and Assassins both gain ATK from their set bonuses. Assassins have the edge here, with higher natural scores as well as a better set bonus. However, Warriors have the option of grinding out Ascended armor, where each piece gets a whopping +84 ATK, versus the cap on Epic gear being +40. The literal cost of getting this godlike bonus is in Shards, as the set costs 720,000.

Sorceresses, Healers and Tanks all get the short end of the set bonus stick. While they do all have their advantages, there's no way around the fact that the ATK stat is far and away the most important. Whether you're grinding for upgrades/currency/crafting materials or fighting a boss, everything gets easier with a higher ATK.

The stat deficient can be compensated for by permanently increasing skills through crafting. The problem with this is, beyond skill level 10, crafting now depends on having an an INT score of 80 to guarantee success [citation needed]. And that doesn't change the fact that an Assassin will still be more effective than a Tank, assuming their gear and skill levels are equal.

There are plenty of ways to farm in LoM, but I've had the most success by using Death Invitations at +15 and running Freedom City Strongholds. Gear drops are relatively rare, but I do get Tier 2 Epics and it's a decent farm for Gold and especially Shards. I would prioritize upgrading your Forge to Level 5 (max), and grinding all excess gear into Dark Cubes, which you'll need for upgrading gear.

Death Invitations can be purchased from Rare Item Vendors:
Ako (Lobby, upstairs)
Haley Ryan (Freedom City, Eastern Resistance Camp)
Monk (Necropolis Desert, after using the Launcher)
Rare Item Vendor (Underground Sewers)