When I played ME3 multiplayer for years on XBox, I had nearly maxed my manifest and played Platinum matches almost all the time. After getting the PC version, I was obviously disheartened by the prospect of grinding several years of progress again. Given the lack of profile transfers, I'm perfectly willing to cheat a bit to restore the legitimate progress I earned back then. 

There's an existing mod almost identical to this, but it only works well with the default store layout, so if you're using Full Experience, it'll buy one of the less ideal promotional packs. I picked Jumbo Spectre Packs because they allowed me to grind promotions for the N7 banner I once had as well.

In order to use this, download Pulover's Macro Creator, run the game in 1920x1080 Windowed, load the preset, ensure 'Loops' is set to 0 so it runs indefinitely, and set up your Control Toolbar or be ready to press F5 once it's loaded. Due to the way the store often loads, there will be a slightly shorter delay when buying the first pack, so wait until you've purchased and opened one pack, then once you're back in the store page you'll activate the macro using the play button or F5. Doing this ensures the macro will run on an even timing for as many hours as you want. I tried to time the macro with slight delays to accommodate for the strange pattern in load times, and have tested it for hours while sleeping/drumming/etc. so let me know if you have trouble on your end.

Additionally, you can safely use the console and type 'GrantMPCredits xxxxxxx' with no concern of being banned. Don't let sad Redditors tell you it's cheating, you deserve the Morbillions of credits required for that sweet Cerberus Harrier X.