Pineapple Express Achievement in Halo: The Master Chief Collection

This achievement is best done on a lower difficulty and can be obtained in about any mission where there is a Phantom Drop ship. This is mainly due to the reason that as these Phantoms arrive, they will drop groups of Covenant onto the battlefield, usually all clustered together. If you’re attentive and watching the Covenant units drop out of the Phantom, you’ll be able to quickly lob a Plasma Grenade at one enemy in the group and the explosion should kill all the Covenant that dropped. 

Generally, the outdoor firefight segment of ONI Alpha Site or the escort segment towards the end of Data Hive work best for this achievement. If needed, you can always turn on the “Competitive Scoring” HUD in the Campaign lobby so that you’ll know exactly when you’ve killed four enemies with a single grenade (it will be indicated by an overkill medal).