Gaptooth Breach where the mine is. As long as you don't reload or switch weapons, you just need to go through the mine and deadeye the weapons out of their hands. (It helps to bring Snake Oil)
If you do a hideout then there will be plenty of enemies and also if you have the Evans Repeater then you will have 22 chances pretty much. Idk if it was a glitch or not but everytime I used dead eye while using the Evans Repeater it would auto reload but I still got the Rank 10 challenge completed.
I recommend doing Pike's Basin with the Evans Repeater, and just remember not to reload. Dead Eye the enemies and just keep aiming for the GUNS and not their hands. Sometimes, if you aim for the hand, you'll kill the guy and it won't count as a disarm.