there is an easy way to get Credits whenever you really need it and have the time to spare, but this trick only works if you have the "SuperNova" addon pack.
1st. buy the HEAVY DUTY DETONATOR when it becomes available. (this weapon is available in the store after buying and installing the "SuperNova" pack. It's free so don't worry.
2nd. In a safe location fire off all the proximity mines, deactivate them, now go collect them. (the DETONATOR starts with 3 mines)
3rd. Sell the proximity mines in the store and the HEAVY DUTY DETONATOR. Then buy the HEAVY DUTY DETONATOR again. When you buy it back the detonator comes back fully loaded. just fire them off and repeat as long as you can.
Each proximity mine s worth 200 credits. for an extra added bonus buy the regular DETONATOR and keep it in your SAFE. When you upgrade the regular DETONATOR the HEAVY DUTY DETONATOR upgrades too. BUT when you sell it back and buy it again it keeps the upgrades provided you have the regular one in your safe. As long as you have that the upgrades stay with the weapon.