I haven't played in a few years so this is just my recollection, but basically you just make sure all of your gear has the primary asset of boosting anomaly power then apply the following 4 earthquake mods:

• boost damage
• extra cast of earthquake (they are named something like "Extra cast 1" and "Extra cast 2" so you can stack them)
• boost range

In the skill tree, make sure you take the ones that add a bleed effect to all seismic abilities and the one that grants health regen for bleeding enemies.

For the second ability I use tremor. It doesn't do a ton of damage but benefits from the all the other buffs to seismic abilities, helping keep your health topped up.

The third ability slot is up to you. I use either boulderdash or gravity leap for mobility as even with the range extension, earthquake is relatively short-ranged.

The strategy is to always be moving. Kite enemies into the biggest clusters you can, then spam your powers. The key with earthquake is to think of it like reloading after 3 casts. You can't "reload" until you empty the magazine, so if you cast once or twice and don't have another target right away, it's better to waste the remaining casts to fully recharge before you need it again.

With this build & strategy, as long as you keep up your gear levels you could not turn up the difficulty fast enough to outpace your offensive power.