I certainly think that the absolute best way to grind credits is on the gold challenge with another friend. One of you plays as a vanguard and the other uses warp/reave so that the vanguard sets off biotic explosions. But often I find myself wanting to unlock more gear quickly but no friends wants to play with me.
That is when I decide to bring out my special vanguard build and play against the geth on the silver difficulty. It doesn’t really matter on which firebase you play, but I find that firebase glacier is the easiest and fastest to finish.
Special Vanguard Build
I have already written a guide to using the human vanguard effectively, and most of what I said still applies. I strongly suggest that you, if you haven’t already, read the article here. The only difference in leveling the vanguard is that you specialize nova so that it only uses half of your barrier. The other build is certainly the more powerful build, so I suggest that you make one of you human vanguards that build and the other this build (male and female are the same class with the same powers).
What this allows you to do is have everlasting invincibility frames. Whenever charge or nova is active you can’t take damage or stagger. If you use nova twice, charge will have come off cooldown and you are instantly ready to charge again. Be sure to mash the buttons that activate the powers. However, there is a tiny time gap between the two nova and charge and nova. Usually that is bridged because the enemies around you are staggered, but if you are facing a geth prime the geth prime doesn’t stagger easily.
If you want to kill a geth prime kill everything around it first. The rockets are very unlikely to hit you, but if you get close to the geth prime it will do a melee attack that has a time gap in which it will stagger you, so after you are done with the nova, even if the geth prime did its attack during your nova animation, you will still stagger. If there isn’t anything around you that isn’t a problem because the geth prime takes a little while between its attacks.
Also if you have some enemies at longer ranges that can shoot at you, particularly rocket troopers. You can be staggered in that brief time in between nova and nova or nova and charge. Be sure to charge and use nova so that you don’t have any enemies at long range, so whilst you use nova maneuver towards cover.
After every second round you will be able to buy a spectre pack. I find that with average teammates it usually takes me around 17 minutes. This technique is so effective that I have completed a silver challenge by myself for the last two rounds and before that with only one teammate for most of the round. This is very unlikely to work through the objective round when you have to complete four objectives, but with target elimination or the one long objective it certainly isn’t a problem. With the latter find cover against the common routes to that area of the map. Only charge enemies that are within the objective circle, the best that can happen is that a geth prime walks in.