How to untie a stubborn knot
On my one of my early big wall climbs on the Prow route in Yosemite, my partner and I had such terrible rope management skills that we often found ourselves completely untying, either ourselves or the haul bag knot, to decluster some horrendous rope mess we had somehow made.
Fortunately, we had a multitool along with some needle nose pliers, which were exceptionally helpful to untie the welded knots.
Hint: it's WAY better to avoid this situation in the first place, you shouldn’t have to be untying like this on any climb if you pay more attention to the rope!
Tips to untie a welded knot
Step on the knot and roll it around a bit on a smooth surface. Maybe that's a very smooth rock, or a floor inside.
If you have a hammer, you can try the same thing with a hammer. Rotate the knot and tap it.
If you don't have a hammer, you can try going caveman on it. Find a smooth rock that's not gonna damage the sling and again, rotate and tap.
image: Vdiff climbing, shared with permission
More thoughts . . .
If you're gonna put a big load on a knot, a butterfly is usually a better choice than a figure 8 on a bight. (And certainly better than an overhand.) The two little “wings” of the butterfly can usually be worked back-and-forth to untie almost anything.
There is a bit of a climbing myth out there that a figure 9 is easier to untie than a figure 8. Opinions on this very, but I've seen some pretty compelling evidence that the figure 8 is actually easier. Here is my article about that, along with a video that tests this idea.
Knots in Dyneema can be extra difficult to untie, and because there is no sheath on the sling, can be easier to damage when you're trying one of these techniques. Use caution. Be ready to throw out the sling if you damage it.
If you are going to use needle nose pliers, start gently and work slowly, so you don't damage the material. Pliers should be a last resort.