Hey,
I have a bit of experience with GameMaker Studio, and I have a background in software engineering, so I think it shouldn't be a problem to implement the systems you need.
Admittedly i've never played Undertale, but im pretty sure i'm familiar with it's battle system :) And well i play a lot of games, so it's not a problem to quickly figure out how it works.
Anyways, feel free to contact me and we can talk more about the project. I'm always up for a challenging programming exercise.