Do I need to have all the eggs in my bag to get the achievement? Or can I just collect and sell?
Baptism By Fire - 15
Defeat 3 Majini at once with a drum or gas tank explosion.
Chapter 2-1, after you take out the oncoming truck Majini will follow and there will be barrels left either side of the bridge. Wait until they're near and blow them up.
There are other opportunities throughout the game but this is the method I used.
For this one I found an easier way; When you face off against the first chainsaw guy where you have to throw Sheva to the other side and come down to unlock the door for you (Shanty Town in demo) have your partner wait for Infected to get up near the barrel and then you shoot the barrel, there are easily 5-6 of em.