If you have the Sinking Feeling brutality for Geras than with the proper AI set up, you can have the AI farm brutalities on the survivalist tower for you.
The AI build I use is 30 points in both Zoning and Runaway and then only equip the Quick Sand ability on Geras and nothing else. Then load up a Survivor tower on Very Easy and select this Gerus build and turn on AI Fighter. The AI settings will make it so that the AI fighter will try and constantly move away from the opponent and use ranged attacks, and since we only equipped Quick Sand on him that's the only ranged attack he has. If he finishes the fight with the Quick Sand attack than it will do a brutality (once again, ONLY if you have unlocked the Sinking Feeling brutality already). The only thing you need to do is hit
after every fight to progress to the next one.
Side note: the AI won't always do a Brutality, if the AI ends the fight with any other attack it will do a fatality instead, or if the Sinking Feeling final hit is a crushing blow than it might ruin the Brutality. Despite all that however, a vast majority of the time the AI will get a Brutality, in the 25 fights Survival tower my AI ends up averaging around 15-18 brutalities.
This isn't as fast as manually doing very easy towers yourself or using a second controller in local vs mode to grind it out, but I prefer this because I can do other things while the AI grind it for me, as long as I'm there to hit
BONUS: This also works as a good heart grinding method. A Brutality gives 5 hearts and a fatality gives 3, if you use the tower consumable that doubles your hearts earned, than its 10 and 6 respectively and could farm around 175-225 hearts per tower just having the AI do it for you.