There are many technique's to kill the harvester, i used my level 31 warrior. But the key to the first form of this battle would be the switch, every time the switch appears run over to it and press it (sometimes you wil need to open your radial menu to activate it.) You will need alot of health potions while doing this because the only heal ability you have is the runic golem's group heal is you picked up the scroll, (which didnt heal me very much at level 31).
while progressing through Amgarrak make sure to check various chests. some will contain upgrades for your golem. If you want a guide for these scrolls (and an achievement) here is a guide: https://www.trueachievements.com/achievement.aspx?achievemen...
Note: you can only get some of them while in certain colours, i just call them lyrium filters.
On the boss i ignored the little skeletons because when you activate the switch most or even all of them die, you can tell the switch has appeared when the area looks like its got red gas around it. the switch is on the north area where the first form takes place (it has a little x on your map). If you are on critcial health, you can just run away and heal (he is probably the slowest boss on the game). He will take a while to go down. After he goes down you will have to kill his 2nd form.
The 2 abilities you will need (if you are playing as a warrior) are:
Caracuss - no damage from skelatons
Second wind - restores stamina
I found the second form really irratating because the ammount of skelatons is intense. My technique for a warrior would be just run around the room until you can activate caracuss. When the ability is ready, use it and go all out on the harvesters 2nd form. When the ability is about to run out, start running again. This will take a few times but i think its the most successful way. Also dont bother healing team members, they arent to helpful in this form. While running around the room if caracuss is ready but you have no stamina, dont worry, just use second wind to restore stamina.
Note: i was doing this boss on hard difficulty so this technique will help you get the grim reaper achievement aswell.
With other classes i would recommend trying different techniques. Also if you have the first tome of the grey wardens, you can change your atribbutes and specializations to try what works best on him. For players struggling with this, there is a glitch you can use with a bow if you want. Here is a link to the site (i didn't use this) : http://www.xbox360achievements.org/forum/showthread.php?t=25...
Not sure if this works but its worth a shot if you can't kill his second form.
If your rogue knows combat stealth he could go invisible and backstab the harvester then run. Dont know if this would work but worth a shot.
When the boss has fallen you will get 25g and you will have completed the Golems Of Amgarrak.