A customer's iPhone 6 was draining battery very quickly, from 55%, it would go down to 0 in around 10 minutes. I knew this was a battery problem.
The screen and battery connector were very easy, but the biggest challenge has to be the iPhone 6 battery adhesive. It is so tough, and the pull tabs rip very easily. I got the first one off (for the first time in my life) and the second one snagged and ripped in half. I had to use a heat gun to get it out, which took around 10 minutes. Reassembly was very easy
Because the first part is very easy, don't be tricked by the adhesive pull tabs, you have to take your time with them, and if they snap/rip, you will need a heat gun, hairdryer or iOpener to help you. Replacement adhesive isn't required, I just used double-sided tape, which makes it easier to reposition.