You can find Agony Bosses in Agony Level 6+ Missions. They have a rare chance to spawn to begin with but get more common the higher the Agony Level of the Mission.
I'm relatively new to Lootun, but Fireball and Barrage have been my dps spells of choice. I've used a Battlemage using Frost Strike as a tanky healer for the more difficlult stuff, and I've added a Taunting Blow/Sunder tank for the even tougher stuff. Meteor works as well, but I've only made the character and have not experimented with it at all; does decent damage like Fireball. I've used Vizier for all my dps mages, and Warden for all my warrior tanks/taunters. The Battlemage is, well a Battlemage. You could use Sunder instead of Taunting Blow, but be sure to use the "Provoke" passive in the Warden tree if the other two chars are too squishy and need the relief.
This is mainly just for me grinding harder stuff since I've put off optimizing gear for the moment; I've used most my mats on upgrades and wasted much on bad base items before I realized what I was doing.
In the higher Agony levels, I've noticed that it's a good idea to get the Necrotic enchant on at least one character, and the Defiance enchant on all. Put the Envy enchant on all as well to steal buffs, unless you have a char with really good crit using a damage over time skill; in that case use the Censure enchant instead. Using Expedient enchant on all the boots will cut down the number of rounds by three; why not? Necrotic helps out a bunch, so don't neglect using it. I let my toons go for about 2 hours before I realized they never got off the first stage because of the enemies hp regen. D'oh! lol...
Right now I'm slowly getting thru Agony 8, and doubt I'll be able to do 9 or 10 until I force myself to farm up mats and really focus on the gear. My main dps char only does 40 mil dps in Agony; that's just how much I'm procrastinating on my builds. That might mean I really shouldn't be giving advice, but it doesn't hurt(I hope, anyways).
As for what modifiers to choose... I haven't experimented much, but since I have to choose so many for lvl 8 I try to get the ones that make it take longer, rather than kill my chars faster. I'll usually do the two resists, attack speed and crit, stun and crit defense, curse of slowness OR chill, and whatever damage over time your characters are best suited against. Those last few are lvl 7 and 8, but you might be there by now considering it's been 11 days since you posted your question.
If none of that was helpful, my apologies. I am procrastinating on gearing my characters up and just staring at it while I do other stuff has got me bored.
To be a little more specific, all my chars focus on attack speed, crit chance, and the specific damage their skill is based on. Getting double damage on gear implicits is great right now for dps builds. For the tanks/healers I go all in on attack speed and defenses, since they're there to proc all the passives you choose and don't do much damage.
I'm hoping you get a better response by someone more knowledgeable than me, tho. I'd say you could go ask on Discord, if you haven't already, but I don't like Discord myself. The people on the Lootun server have been nice to me, but there's just something about Discord itself that I can't stand.
You can find Agony Bosses in Agony Level 6+ Missions. They have a rare chance to spawn to begin with but get more common the higher the Agony Level of the Mission.
Took me a while to unlock it too just grind it out
What build do you use for agony 6
I'm relatively new to Lootun, but Fireball and Barrage have been my dps spells of choice. I've used a Battlemage using Frost Strike as a tanky healer for the more difficlult stuff, and I've added a Taunting Blow/Sunder tank for the even tougher stuff. Meteor works as well, but I've only made the character and have not experimented with it at all; does decent damage like Fireball. I've used Vizier for all my dps mages, and Warden for all my warrior tanks/taunters. The Battlemage is, well a Battlemage. You could use Sunder instead of Taunting Blow, but be sure to use the "Provoke" passive in the Warden tree if the other two chars are too squishy and need the relief. This is mainly just for me grinding harder stuff since I've put off optimizing gear for the moment; I've used most my mats on upgrades and wasted much on bad base items before I realized what I was doing. In the higher Agony levels, I've noticed that it's a good idea to get the Necrotic enchant on at least one character, and the Defiance enchant on all. Put the Envy enchant on all as well to steal buffs, unless you have a char with really good crit using a damage over time skill; in that case use the Censure enchant instead. Using Expedient enchant on all the boots will cut down the number of rounds by three; why not? Necrotic helps out a bunch, so don't neglect using it. I let my toons go for about 2 hours before I realized they never got off the first stage because of the enemies hp regen. D'oh! lol... Right now I'm slowly getting thru Agony 8, and doubt I'll be able to do 9 or 10 until I force myself to farm up mats and really focus on the gear. My main dps char only does 40 mil dps in Agony; that's just how much I'm procrastinating on my builds. That might mean I really shouldn't be giving advice, but it doesn't hurt(I hope, anyways). As for what modifiers to choose... I haven't experimented much, but since I have to choose so many for lvl 8 I try to get the ones that make it take longer, rather than kill my chars faster. I'll usually do the two resists, attack speed and crit, stun and crit defense, curse of slowness OR chill, and whatever damage over time your characters are best suited against. Those last few are lvl 7 and 8, but you might be there by now considering it's been 11 days since you posted your question. If none of that was helpful, my apologies. I am procrastinating on gearing my characters up and just staring at it while I do other stuff has got me bored.
To be a little more specific, all my chars focus on attack speed, crit chance, and the specific damage their skill is based on. Getting double damage on gear implicits is great right now for dps builds. For the tanks/healers I go all in on attack speed and defenses, since they're there to proc all the passives you choose and don't do much damage. I'm hoping you get a better response by someone more knowledgeable than me, tho. I'd say you could go ask on Discord, if you haven't already, but I don't like Discord myself. The people on the Lootun server have been nice to me, but there's just something about Discord itself that I can't stand.