[Update 11-11-2016] The Breach server has updated with the following small hotfix changes:

• Provides the Premium "NOSCOPE" Sniper Rifle in either A or S grade. Hipfire gun that actively blocks scope use.
  
• Improved the chances to find Expansion Items in packs (Enhancer, Elite, Ripper). Added the chance to find them in Victory Packs (Challenges).

So the topmost reason for your need of Credits may be the recently added achievement Excess Baggage Fees.

Excess Baggage Fees
Breach: Complete a server using 6 Expansion Items at once.

Expansion Kits are found in at least an Enhancer Pack, which costs 15,000, but can also be part of Universe Packs - those are packs that I can confirm drop Expansions, there may be other ones too. This is what the items look like:

The easiest method to farm Credits once you have completed all Breach servers and have run out of normal ways to earn them is to beat Friend Challenges. This can be achieved if you ask a friend to send you easy ones such as beat S01_LAVAWALL in \\NETWORK_00 in 15 seconds.

Upon completion, the rewards will be as follows:

The Victor:
* 13,000-15,000 Credits
* Victory Pack

The Loser:
* 3,000-4,000 Credits
* Backup Pack

You can use the items from packs to sell them for even more Credits, the most valuable ones being Weapon Attachments (3,000 Credits) and Nuke Viruses (3,000-5,000 Credits). Refer to the video below for a demonstration of how quickly you can send and beat a Friend Challenge in S01_LAVAWALL.


Note that if you fail a Friend Challenge 3 times, you will become "The Loser" and will receive a Backup Pack while your friend will earn the Victory Pack.

Unfortunately, a user can only send between 3 and 5 Friend Challenges a day. These can be refilled with Firewall Keys which if you have finished most levels already will be hard to come by. A Firewall Key costs 50,000 Credits so if you want to earn Credits, then this seems counter-intuitive.

That's where an alternative Steam account comes in place. If you have another account (or a family member who isn't interested in Deus Ex), you can share the license and set up Family Share to allow them to access Breach for free. With their fresh account, you will have LOTS of Credits to purchase Firewall Keys with and send your main account Friend Challenges.

You will only need to complete the tutorial and you will be able to buy at least 5-10 Firewall keys, which is worth 15-30 Friend Challenges for your main account.

Thanks to JieXY for the idea of alternative accounts!