I agree with removing the Taptic Engine first, before pulling on the adhesive strips. It really, really helped. As long as your iPhone6s Taptic Engine looks like the one in the picture ( wide, and almost width of the battery. Mine also had black tape over it too, for some reason). This wide Taptic Engine only has the two screws to remove - no power connector cable! ( it uses contact pins on the underside to power the Taptic Engine ).
Good News - Bad News: This was my very first time,ever, in trying to repair any electronic device. Yes I was 100% successful in replacing the battery in my iPhone 3GS. Thank you iFixit for the parts. BUT---- No thanks to iFixit for the not very helpful instructions, dark photos and no close-up shots. Also, the iFixit video is utterly useless. THANK GOD I found a iCracked.com video on YouTube (" Official iPhone 3G / 3GS Battery Replacement Video & Instructions - iCracked.com ). It gave me the confidence I needed to actually do the repair. The instructions were very clear and the video was also very precise. Especially about how the #3 ribbon cable connector works - Very Important. It even helps you re-assemble the iPhone in the reverse order. I am a complete idiot when it comes to this stuff, and even I was able to replace the battery and have the iPhone boot-up in perfect working order when I was finished. iFixit got my money for the parts which is terrific - but the instructions from them were lacking.