In this guide, you will come to know where to farm for Grim Fury in Warframe.
Drim fury can drop from two places, either as a low drop chance drop reward from killing power fists or a completion reward for completing Cetus/plains of eidolon, Bounties. If you plan on farming power fists to get the mod, then running the endless or exterminate missions on Saturn is recommended.
The specific Cetus bounties you have to run are levels 10-30 as they can have grim fury as one of the items in the reward pool. If you don’t see the mod in the pool, you can wait until the reward pool refreshes; you can see the time at the top of all the bounties.
The mod in the bounties doesn’t have the lowest drop rates, at least out of the items in its reward pool, so it shouldn’t take you ridiculously long to get the mod unless you are unlucky.