I have only tested it to work for the steam version. Just use any of the usual memory search tools (CE) to search the HP number of any character during battle (select 'double' instead of 'integer').
Make changes of the HP amount by taking damage and do a 'next search' and you will be left with a list of addresses of that value. Just set a number to those addresses (eg 999) and go back to the battle and your character will have that HP value until taking damage again. Then just set that list of addresses to 999 again and you are basically invulnerable.
You may also edit enemy HP to 1 as well and just KO them on the next hit! Final boss of Prologue does not take damage so there's no need to try and kill him, just stay alive and run to the escape point. Only 1 member need to do so within 5 turns and you will get to 3-star the mission easily.
Hope this helps until the dev team makes a mod!