Grim Reaper Achievement
Defeated the Harvester on Hard or Nightmare difficulty
I have good news and bad news for you. The good news is that you can switch difficulty right before The Harvester fight (after you find the sixth note) and get the achievement. You CANNOT switch the difficulty once the fight begins however. If you had thoughts about getting the Harvester within one hit of death changing the difficulty, think again. The bad news, the Harvester is insanely hard. The easiest character type to beat the Harvester with is a Mage. With a Warrior or a Rogue, struggles will ensue.
With the Mage, you want an Arcane Warrior. Some of the spells that you want are flame weapons, crushing prison, group heal, revival, haste, and miasma. With the Warrior, be a tank. With a high enough number of hit points, you can take anything. Just have plenty of health poultices. With the Rogue, you'll have to be crafty. Attack the Harvester from behind as much as you can. If you have dual wield and two good weapons, use them. Otherwise, stick with one powerful weapon.
Prepare early. Find all of the notes for your golem and get it fully upgraded. Set it's tactics to Ranged and make Group Heal it's top priority. Set the tactics of the two dwarves to Defensive. Set three of Brogan's tactics to use "Taunt" and "Grievous Insult" and to use a stamina poultice when that drops below 25%. This will ensure that the Harvester is usually targeting only Brogan. You'll spend most of your time in the battle with Brogan selected downing health poultices, so make sure you have plenty of health poultices left. I had about 220 in my stockpile when starting the battle. Anytime someone's health drops to about half, switch to them and heal unless the golem takes care of them first.
The first form is the hardest. This first phase of the fight is really about how quickly you can knock down the Harvester's health while avoiding the "yellow" and "orange" skeletons. Hack away as best you can and heal anyone that needs it. During the first form, skeletons will start to appear around you. The Harvester will basically stay in place though. You want to start the second phase with as few skeletons as possible since the Harvester will just summon more. One good thing you can do to get around the skeletons is to use the switch on the right side of the room as you enter. If you click on it, the skeletons will disappear. To click on it, hit (or hold depending upon your set up) , get the switch in the center of wheel, and push . Be warned though, more will appear a few seconds later though, so get your shots into the Harvester quickly. At certain points, you'll hear one of the dwarves yell that weapons are no longer effective. If you can, switch to the golem and cast chain lightning. If you find yourself being overwhelmed, as the sole survivor, and you can't get to the switch, then start running around. Get some distance between yourself and the Harvester and chip away at the skeletons. The most important thing to remember with the first form is that you keep the golem alive at all costs. If the golem goes down, you will too, eventually.
Once you defeat the larger form, the Harvester will become the little crawling thing you've seen in the distance. It'll jump around to certain points in the room to spawn skeletons. Continually attack it as best you can. Move around when you get overwhelmed. At least with this phase, all of the skeletons will be standard ones. When you're pretty clear of skeletons, hammer away on the Harvester. The second form is much easier than the first.