After some testing with the Great Leveler with my Tempest Ranger, I made some discoveries I'd like to share, so that you can use this ability better.
First, let's look at the tooltip:
The red marked text doesn't match my tests at all (using strawmen in guild cargo hold for rough estimation and some quests for confirmation). Instead:The Great Leveler: Melee attack. 3/4/6[W]. +4/8/16 Critical Threat Range. This attack creates a shock wave that affects all enemies in a short narrow line. The length and width of the line doubles if Adrenaline or Quick is active, consuming those effects. Applies Trip (DC 20 + Highest of Dexterity/Strength/Constitution + Trip/Tactics DCs). 30 second cooldown.
Without quick/adrenalin: The AoE is more of a square, 3x3 character models wide, and you're in the middle of one side. You hit targets directly left and right of you, as well as targets that are about 2 models ahead of you.
With quick (no adrenalin tested): The square becomes a line, about 3 times as long.
The ability isn't bad at all, but as it works differently than the tooltip suggest, I'm sure many people thought it's very unreliable and not worth the time. Though using quick and immediately Great Leveler actually functions reliable enough and when you know how the AoE actually is, you can make the Leveler work well enough with some practice.
Hope you can make it work and SSG works on either the AoE or the tooltip.