Depends on "narrative" of a fight. Sometimes general battle music kicks in, sometimes lyrics made fight more exciting. When fighting someone known for theirs might(even if only you know them), by sound you can accent how hard it was, or vice versa. But it my opinion. Music is a tool for show your creativity or emotions from that fight.
should i use music that isnt from the armored core franchise?
Yes but i do like ac music hehe
Depends on "narrative" of a fight. Sometimes general battle music kicks in, sometimes lyrics made fight more exciting. When fighting someone known for theirs might(even if only you know them), by sound you can accent how hard it was, or vice versa. But it my opinion. Music is a tool for show your creativity or emotions from that fight.
if it fits with armored core gameplay then by all means!