Which single player levels are best for grinding eggs?
Solution 1:
I've finished the campaign, so I feel comfortable saying that the eggs are all fairly balanced at roughly ~200+ per normal level. It seems that if you just go through and do each weapon challenge on a level it will fill up your eggs quite well after completely clearing all challenges on a normal level or two. You're probably also going to have to replay levels anyways to find the sardinium you need to upgrade them anyways.