I just want to share my personal route for grinding materials to craft Mega Barrel Bombs. It's way more efficient to have Botanist Level 4 like I do in this video. I usually gather other materials along the way, but for the sake of the video I only gathered the required mats for Mega Bombs.
The materials should respawn by the time you finish the route, just fast travel back to Camp 11. Rinse & Repeat.
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How to craft Mega Bombs:
1x Gunpowderfish Scale + 1x Large Barrel Bomb
OR
1x Devil's Blight + 1x Large Barrel Bomb