Assume Command

If you're looking to complete A Magnificent Contribution, Assume Command and/or It'll Nether Happen from the scratch be prepared for a pretty long grind.

These achievements require you to contribute [currency=1342] to the respective buildings 50 times.
1 contribution requires 100 supplies so you'll need 5000 supplies for 1 of the 3 buildings (=achievements) and 15.000 for all 3 of them.

If you've several toons, check if you've already gathered some supplies / made progress on these.
Attribute one of your toons to each achievement so you can work on all 3 achievements at the same time.

In order to gather [currency=1342] reliably you can do the following things:

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• Do the dailies on the Broken Shore with war mode on. Each quest rewards 28-29 supplies.
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• Kill the rares and collect the treasures along the way. On average those reward around 5 supplies each. I wouldn't bother proactively farming them though if you don't want to burn out early.
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• If the Command Center is up, look for [mission=1698]. That mission grants 500 supplies and will save you 2-3 days worth of dailies. You can also use the WoW app for that.
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• If the Nether Disruptor is up, buy Nether Portal Disruptor in larger quantities and search for [npc=120751]. They are scattered around the Broken Shore and do in fact respawn. Each rare spawned from the portals rewards up to 20 supplies.
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You can't yield more than 1000 supplies so I'd suggest handing them in once you finish your daily session.

Depending on which building is available during your time, completing these achievements will take about 2-3 weeks if you work towards them on 3 different characters on a daily basis.