I've joined a survivor twice now in the past couple of days and have been evicted 20th and 19th.
There was really no reason the first time besides apparently someone else had a better alliance than me, but the second time I had the lowest challenge score for that competition.
How do I get better at these challenges?! It just seems everyone is so much better than I am.
Just start doing the practise links for challenges :) and make sure you join with 100% health so you have more time in the challenges... You can get more health by playing fastings :)