Brawler's Guild Guide - thanks Zendai.
Rank 1-6
Level of difficulty: faceroll (just learn the one shot mechanics)
Gear requirement: none
Rank 7
Level of difficulty: faceroll (just learn the one shot mechanics)
Gear requirement: 480-490 ilvl
Bosses
GG engineers: Pretty tough with enrage / self healing needed. They share health so use judgment glyph / roll censure on both targets. Use LoH efectively to minimize hp/globals used to heal. Rockets also friendlyfire so focus the death ray midget so rockets splash on him.
Dark summoner: Unforgiving to melee, if ghost gets near you without fear, you die. Aim to kill 3-4 ghosts and nuke the shit out of him in between. Good rng (so you can keep more than 1 ghost feared and whack the boss at the end) helps a lot. Can be frustrating when you have one ghost up and other spawns behind you.
Battletron: Gear check. Don't blow more than 2 orbs so healing requirement is minimized (I guess you could bubble at start to not get damage from explosions and LoH later at fight). Other than that, there's nothing else than doing enough dps to beat enrage.
Meatball: Easy. Collect orbs and blow cooldowns at the last 30secs of the fight.
Rank 8
Level of difficulty: hard to very hard
Gear requirement: 490+ ilvl
Bosses
Epicus maximus: kinda obvious fight, avoid shit and interrupt blue crush. Tight enrage.
Zen'shar: same as above, without interrupt and some aoe at the start.
Millhouse: three letters: RNG. Depending of your gear, you need 3 or 4 of the first orbs on you. Then it's just dps race. Blow cooldowns at 100 stacks, when they align with trinkets. Kill totems you can't get only when you can one shot them with judgment - boss can get stacks as long as he doesn't get 100. Obviously don't do this during cooldowns.
Disruptron: I guess it's doable with ~500 ilvl gear, godlike skills and good rng (boss needs to move to corner when phase 2 starts) but don't have the energy to go for this myself. There's enhancement shaman killshot posted on these forums, so it's doable. There's also some levitate kills done, but afraid they fixed this and it's no longer possible.
GG Engineers - May require Seal of Insight Glyph and Divine Storm glyph for extra healing.