Alright I'm going to take a crack at this and try and make a solution for people post power glitch who are still endlessly grinding for this; this is my first time so please be kind
UPDATED: 6/24/17 made a few tweaks and comments on what worked best in getting the achievement
Shout out to: Active, DRF Patch, D3LTA, and katerfone for making this possible
As stated before your best bet is to complete this achievement with the engineer. Your looking at about 75,000 scrap to do so.
I've been using a method on horde using the map war machine. To accomplish this your going to need this team make-up:
2x-Engineer running maxed out (Sentry Cost & Capacity, Repair Boost, Build Cost, Carry Speed)
2x-Scout running maxed out (Deposit, Health Boost, Pickup Distance, Speed, Energize)
1x-DPS Solider (Grenade Plants, Damage, Capacity Resupply and Assault Rifle Damage)
Put the fab near its spawn by the station cafe. Split into "South Entrance" and "Platform 3" (most of the time referred to as Blue and Gold sides) teams with an engineer and scout on each side. One engineer is going to need to stay with the fab to mass produce sentries and barriers. The DPS solider will cover the weaker of the two sides. Now in the back of the map you will see a giant COG right in front of the two archways and a smaller COG past it. Your going to place a barrier in the archway and a sentry BEHIND the giant cog. Now before the wave starts each team takes position on the smaller cogs to spawn push all enemies into the the platform rooms. Using this method level up your fab by making new sentries and letting them destroy old ones. Scouts run power back and forth etc etc. eventually you'll have enough sentries to spawn trap and have some reserves for the bosses. Using this method inconceivable/ ironman difficulty is not an issue at all. Keep making and using the inconceivable XP bounty (1200) and you'll make around 4,000 per run.
As far as what pack to get I believe depends on your situation. If your missing cards and need to unlock them go with horde booster packs or operations. If your in need of scrap I would go with elite or community packs that seem to have a much higher chance of dropping a legendary but almost guarantee epics (elites definitely guarantee and epic) especially to level up the epic skill cards. Probably for the greens and blues you can go back to taking your chances on horde booster packs.
Hopefully another and better solution can come out to make this even faster. But until then get ready for the grind fest. Best of luck to all of you!