Nerf Scrapbots (10 player)

Prerequisite: It will be extremely difficult (rare) to get this achievement if you do not at least have DPS to kill a heart as chaining 2-3 heart phases back to back is highly recommended, if not required, to reduce RNG failures. The objective demands a a fairly rare occurrence: 20 scrapbots up at once with a boombot, all within close proximity of each other. Kill the boombot and its explosion will likely destroy all the scrapbots near it all in one shot. There is a 12 second time limit on this so it's not possible to kill 5-10 scrapbots using this method during each heart phase.

Note: RNG is a painful factor in getting this. If your DPS is exceptional and can get Must Deconstruct Faster basically every time, then you should consider getting a 3rd heart phase in a row. The way I suspect that this works is 3-5 waves spawn each add phase. They can be either: A) 1 Pummeler, B) 5-10 scrapbots, C) Boom Bot + a few scrap bots. You can probably imagine that you can easily get screwed over on RNG. If you are purposely going for this, you will /facepalm when you see Pummelers spawn. You will likely need many attempts to get this achievement.

Strategy

Positioning: Tank XT between 2 piles either on the left or right, against the wall, rather on the stairs. This makes it much more likely that bots spawn in the far 2 scrap piles unless Blizzard makes a change.

Special assignment: Assign people to leave AoE slowing effects down near the far scrap piles to prevent the first heart phase spawns to not be so far ahead of the 2nd heart phase bots. Shamans (healer pref, as there's nothing to heal when chaining heart phases) and Hunters can quickly ghost wolf or aspect of the pack over once the first heart phase begins.

Process: Get XT down to 75% to trigger the first heart phase. Damage the heart down until it's 5% or less before it goes back up, but don't finish it off. Get XT down under 50% and trigger a second heart phase ASAP (should already be under 50% if you get the heart to 1% - heart is 1.5mill and XT is 5mil, getting heart to 1% will mean 33% less HP on XT, 8% more than needed to trigger the next heart phase). Hopefully, RNG was nice enough to spawn at least 20 scrapbots, 1 boombot, and no pummelers. Everyone, but the MT and MT healers, should now focus on bots, using a combination of whatever you have at your disposal: piercing howl, thunderstorm, frost nova, death grip, charge/intercept stun, chains of ice, etc to control the pack of bots (preferably relying on the more reliable abilities). If they are not controlled well and get close to the "converge point" (where the funnel together close to XT) without being grouped up well, you can possibly have a druid taunt XT and cat sprint kite XT along the outside walls to buy a little time. Beware that you still have to deal with light and gravity bombs.

If all goes well, congrats, you have just earned an achievement for playing the add spawn lotto and getting <scrapbots> <scrapbots> <scrapbots> <boombot> and playing with XT's toys and getting them to all go boom at once.

Addendum: Despite what was written on Wowhead's XT page, boombots *don't* follow normal aggro tables and beeline for XT. They do not self destruct or get sucked up by XT. You must kill them and they explode on death and instantly kill near scrapbots no matter how much health they have. They have 12.6k hp and have a 10 yard explosion radius.

The basic solo strategy is drag the boss up on top of one of the junk piles and DPS him to below 50%. You jump down into the path of the adds whilst the boss undergoes two add spawning phases in a row. At the end of the 2nd phase, you try to kill all the boombots that you possibly can.

As a veteran of 107 attempts with a maximum score of 18, not 20, I'll offer this advice.

Do NOT attempt this on a character without an aggro wipe, or some way to reset the encounter, otherwise Insanity will ensue.

Do NOT attempt this if you've already developed a pathological hatred of Blizzard RNG, otherwise Insanity will ensue.

If the boss spawns even one set of adds from the pile closest to him, you've failed.

If the boss spawns even one big Pummeler instead of a group of bots, you've failed.

If the boss bugs and doesn't enter a spawn phase, you've failed.

If the Boombot isn't grouped with the straggling Scrapbots so at least 4 die in one explosion, you've failed.

Basically, if anything except PERFECT LUCK occurs, you've failed.

Macro a ranged spell to target the XE-321's directly, and also to apply any movement boosts automatically, as you may need to run from one pile to another.

Above all else, if you cannot tolerate RNG, do NOT attempt this.

I attempted this around 50 times as a Shadow Priest and finally found a strat that works for classes with no cc for the adds.

First, take the boss to the very top of the scrap pile to the north west corner of the map. DPS him down through all of the heart phases. During the first two heart phases, don't move or damage the heart. Also, don't worry about the scrap bots that make it to him, they shouldn't be a problem. When he goes into his final heart phase at 25, leap down off of the scrap pile and with luck there should be just enough adds closing in on him to get the achievement.

Something to note for Shadow Priests:
Spec into Spectral Guise. If there aren't enough adds out just stealth and he will reset.