Harvester 101

All harvester mobs use the exact same tells, from the world boss harvester, to a certain story line boss, to the regular old harvesters slithering around Cold Harbor. To learn to beat them consistently, learn their tells first. The place to learn this is against the Harvesters in Cold Harbor:

Step 1: Move
The Harvester AI (and generally all AI in the game) is remarkably stupid. When looking at a Harvester, a human being cannot possibly comprehend the magnitude of stupidity looking back at them. The most dangerous of this enemy's attacks is a cone of bluish, translucent arms that travel along the ground in the direction the Harvester thinks you're in. Conveniently, if a player is to continuously move to the right or left keeping the Harvester under their reticle, the Harvester will be absolutely baffled by the astounding maneuver and miss you every time.

Step 2: Hit It In The Balls
Now inevitably the Harvester will get flustered by the players impeccable command of their WSAD keys and use an attack that will drain substantial amounts of your health and convert it into floating balls that should they float into the Harvester will heal it. There's ways to prevent this from happening to begin with by using miss chance debuffs and dodge chance buffs, buff if you have neither, no worries, just hit it in the balls. Each ball you destroy will restore your health and deprive the Harvester of its healing mechanic. The best way to hit the balls is with a bursty AoE such as Volley for bow users or Impulse for d-staff users and so on. Doing this you'll find yourself back at full health in no time.

Step 3: Interrupting is Proper Etiquette
You'll notice that the Harvester has a very obvious casting time tell for its cone attack that you've already learned to avoid by casually moving to the left, but it also has some projectiles it likes to throw at you when it isn't doing that. Be sure to block them, or if your a Dragonknight, Templar, or Shield user, throw up reflects to ping-pong them back into its face. As for the cone attack, bash it in the noggin, thwack it with a Venom Arrow, beam it with Force Shock, or generally anything that'll bring a halt to the cast. You'll now have the idiot stunned and off balance for a quick power attack to put it into another stun. Use this reprieve from its attacks to beat on it without abandon.

During the fight you'll revisit these three basic steps at varied intervals until you ultimately convert that dirty slag's HP into your XP. Happy Hunting!