There are two easy ways to get this achievement. Both ways are very easy to do.
Stand on a helicopter drop point, and request a weapons drop. Choose a manual explosive (Quakie Grenade, or a Satchel Charge). Throw them all on the ground (don't detonate them), then request another weapons drop to reload on explosives. Repeat until you have at least 30 explosives on the ground. Switch to an explosive weapon like a rocket launcher (holding to set off the charges doesn't count) and shoot the explosives on the ground from a safe distance to cause a huge chain reaction.
Another easy way is while in the area for the third beacon in Hope Springs. There are tons of explosive barrels down in the area for you to shoot within 30 seconds of one another.
Mounted guns or vehicles will earn you this achievement easily if you just spam them. You can get this during the tutorial however. At one point there's an empty swimming pool that will be filled with lots of Freaks followed up by you being told to pick the grenades near it up and to kill the Freaks in the pool with them. If you throw them where there's no Freaks and keep spamming RB to throw them while standing on the spot where the grenades spawn then you'll be instantly restocked with grenades every time you throw your last grenade. It's an easy way to get this done and dusted very early on.
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