I advise you grind out Hangar Bay, Checkpoint 2 a few times to fully upgrade your Assault Rifle (will kill most everything quite easily) and moderately upgrade your Beam Cannon (for armored enemies and bosses), which will allow you to grind the X1 and/or final boss much more easily.
Recommended grinding spots (based on time/effort/enemies versus credits profit):
Hangar Bay, Checkpoint 2
Pays out 900 credits when you obtain a Veteran rating. This short level consists of only 4x Turian Troopers, and 4x Turian Snipers. All enemies are easy, in a small room, and there is plenty of cover provided for you. Replay the checkpoint from the battle analysis screen and you'll be placed just outside the room to re-start the process. From the starting balcony, you can take out three enemies without much danger, using a Sniper Rifle. 900 credits for a Veteran rating won't be beaten until X1 Wing's Checkpoint 4 (1,300 credits).
X1 Wing's Checkpoint 4 and 5
Checkpoint 4 pays out 1,300 credits and Checkpoint 5 gives you 1,200 credits (both for Veteran). Whilst these amounts are a significant increase, they are much longer than Hangar Bay's Checkpoint 2 with bigger areas, more enemies, and a wider variety of enemy types.
Colosseum, Checkpoint 1
The X1 boss battle pays out 1,800 credits on Veteran -- the same as the final boss battle does. But with a moderately-upgraded Beam Cannon, the X1 can be beaten very easily and you won't even need to take cover. In the second phase the fight, run to get some distance from him before gunning him down again.
Desert Planet, Checkpoint 1
The final boss pays out the same as X1 will, but has more health and most of her attacks mean you have to take cover. However, a max-upgraded Assault Rifle or Beam Cannon can deal with her; you just have to change cover quite often and wait for her to re-materialise in different areas. HOWEVER, there is an unskippable cutscene ending this level before you can replay the checkpoint, which can be a bit annoying to watch over and over. Especially since the time spent watching this each time could be spent on doing a quarter or half of Hangar Bay Checkpoint 2.