If order matters, yeah, it gets really messy. Here's how I would go about it:

Lets count the number of ways that n gems could fuse into one, and call this f(n).

f(1) = 1: If there's only one gem, there's only one valid fusion sequence: no fusions at all. f(2) = 1: Two gems can only fuse with each other. f(3) = 4: Either all 3 gems fuse together at once (1 way), or 2 of them fuse and then the third joins (3 ways).

We could go on, but with four gems the number of ways they can fuse starts to get hard to calculate by hand. Instead, we'll try to figure out how to calculate this function f.

We're actually going to solve for a slightly more complicated function g(n, k), which is the number of ways n gems can fuse into k fused entities. This may end up being easier to calculate, and if it is, we can plug in 1 for k to get the number of ways they can fuse into 1.

Instead of thinking about the number of sequences in which gems could fuse, let's think about the ways a fusion of n gems could defuse , which should be the same number, since each defusing sequence is just a fusing sequence in reverse.

When the giant giant woman defuses for the first time, she can defuse into as many as i gems, and as few as 2. For convenience, let's assume these gems are numbered. After this first defusion, Gem 1 will either be fully separated from the fusion, or part of a slightly smaller giant woman. Either way, she is now made up of j gems. (If she's fully defused, j is 1.)

Say we have n gems that all fuse into k entities, through some sequence of fusions. We care specifically about the case where k = 1, but solving the more general case may turn out to be easier.

In the final step of this sequence, all the remaining gems (which can themselves be fusions) need to fuse together. This number could be as little as 2 or as much as n. Call this number k.