Because of the Taptic engine assembly, it is almost impossible to pull the adhesives at the optimal, less-than-60-degrees; even when I removed the two-piece assembly, two of the adhesives — middle and the one adjacent to the battery connector — on my wife’s 4-y/o 6s Plus broke.
I would definitely recommend either iFixIt’s “Opening Picks” or “Plastic Cards”; an old driver license (CA) or drive license would work. Just be aware that the edge of the plastic will show some wear and some sharp edges. I was able to loosen about a quarter of the batter from the bottom left, enough to lift (and slightly deform) the old battery to be able to use the tweezers to grab the broken end and pull along the long-edge of the old battery to remove.
Finally, do a check by positioning the battery, with the adhesive film still in place, and make sure the battery connector is lined up with the port on the iPhone: use the spudger to gently crease, if needed, the battery connector “flexible arm” so that the connector lies flat.
Even with a four-year-old 6s, the battery adhesive came off cleanly. Keeping it at a shallow angle as possible, greatly helped by removing the Taptic engine, makes it very easy to remove the adhesives without really much effort. Do it one at a time, however; I’ve seen videos, one by iFixIt, in which more than one were pulled out simultaneously, which resulted in two of the strips breaking off.
This is the first time I opened my 6s since it was purchased new four years ago. Even without heat, constant and firm works enough to be able to squeeze the flat edge of the spudger. From that point and without any additional pulling from the suction, twist the spudger gently to “walk” it open on both sides.