mitigate different taxonomies

There's a document with 30K pairs of "Category in taxonomy 1" and "Category in taxonomy 2". One of the taxonomies is my company's. The other one is the client's taxonomy.
I need to find a way to map the 2 taxonomies efficiently (create a crosswalk between the 2 taxonomies). E.g.:
- A in Taxonomy1 → B in Taxonomy2
- C in Taxonomy1 → Part of D in Taxonomy2
- E in Taxonomy1 → X,Y,Z in Taxonomy2

Any idea here, guys?

Really appreciate it!