you have to do it in steps.
mybb to phpbb 3.1.12 convertor for mybb1.8 to phpbb 3.1
and convertors how to
then upgrade the 3.1.12 to 3.2.5
upgraning from phpbb 3.1 to 3.2]
FIRST, BACKUP everything, files, databases etc. etc.
make a new backup at the success of each stage so that if it goes south you don't have to start from the beginning again.