RESIDENT EVIL 5
50 Achievements
1,000
40-50h
Baptism by Fire
Defeat 3 Majini at once with a drum or gas tank explosion.
15
How to unlock the Baptism by Fire achievement in RESIDENT EVIL 5 - Definitive Guide
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.
These barrels are prominent throughout the game, but there is a large concentration of them in 2-1 in the area just before the Chainsaw Majini attacks you. While being attacked by the Chainsaw Majini, the lesser Majini will spawn indefinitely making getting three together no problem at all.
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.
At the end of the first mission you'll face an large swarm of enemies and an executioner. When the executioner smashes down the gate you'll find an explosive barrel to the right. Simply run past it and the enemies will follow.
Wait until you have at least 3 opponents and shoot the barrel.
Note: The barrels have a large range. Don't get caught in it.