There is a permitted method for credit farming, however I would not advise it. There is nothing better than gaining a rank that you have achieved through had work and determination. But, I digress.
On the level ONI Swordbase, if you start by killing a grunt at the beginning to get a plasma grenade, and then run past the rest killing as few as possible until you reach the underground passage about 50-60m from the start. Here you will find the Target Locater, pick it up and aim at the incoming group of enemies before stepping back as far as possible.
This will make you hit a checkpoint. Now as soon as possible look up and throw the plasma grenade, this will kill you and restart the checkpoint. Find a method to hold down your grenade button, and the left analog-stick to make you look up at all times.
Now you will constantly spawn and get a killionaire before dying and respawning again. This will make you race through the commendations and gain credits for each tier. This is AFKable and fast.
Here's a video that will explain it. It's not the best quality, but still works!