This guide will tell you where to farm for Brutal Tide in Warframe. Go through the guide and follow the instructions carefully to farm it.
The brutal tide can drop as a drop reward for killing certain infested enemies found in the infested missions but tend to spawn more often in dark sector missions.
The two infested enemies you will need to look for are toxic ancients and boilers, which can spawn in just about any infested mission type but tend to spawn more often in dark sectors. It is recommended to run survival or any endless dark sector missions to farm them.
This mod has the same fairly low drop rate as any other stance mod that drops from enemies, so you have to prepare yourself for the grind if you plan to farm it out. You can always buy it with plat off someone if you have enough since the mode should be fairly cheap now.
It usually takes some time before the toxic ancients or boilers start to spawn. For the endless missions, you have to stay in the mission before the enemies start to spawn.