Unzip the APK, you should find a lib/armeabi-v7a folder (or something similar depending on device architecture).
Inside this folder you should find a libmonodroid_bundle_app.so which needs unpacking:
Using a Python script stolen from this Github Gist:
from elftools.elf.elffile import ELFFile
from zipfile import ZipFile
import gzip, string
from io import StringIO, BytesIO
data = open('libmonodroid_bundle_app.so', "rb")
elffile = ELFFile(data)
section = elffile.get_section_by_name('.dynsym')
data.seek(0)
data_read = data.read()
for symbol in section.iter_symbols():
if symbol['st_shndx'] != 'SHN_UNDEF' and symbol.name.startswith('assembly_data_'):
print(symbol.name)
dll_data = data_read[symbol['st_value']:symbol['st_value']+symbol['st_size']]
dll_data = gzip.GzipFile(fileobj=BytesIO(dll_data)).read()
outfile = open(symbol.name[14:].replace('_dll', '.dll'), 'wb')
outfile.write(dll_data)
outfile.close()