DC 15 to climb a free hanging rope? This is something children routinely do at high school, and Soldiers routinely do during basic training:
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I dont see kids and soldiers falling off ropes 75 percent of the time they try.
An adult human in good physical condition (Str 10) automatically succeeds in climbing a free hanging rope. The DC (if any) should be no higher than 5, with only failure by 5 or more resulting in a fall (meaning that only weak individuals with no training have a chance of falling).
As long as you're not morbidly obese and are capable of pulling your own body weight up, you just do that, and then scissor lock the rope with your feet, and repeat.