Hi! Personally, I use game guardian when modifying the values: HP, MP and SP. Also to modify the mount speed (around 300-350). Even so, if I try to modify values such as: skill points, combat statistics, items, etc. I'm starting to have glitches in the game.
When failures occur, the solution that has worked for me is to delete the data and cache of the game. Then I download the data again, with my previously linked account. And I can use game guardian again to modify safe values (Hp, Mp, Sp). That way, I can finish dungeons above my current level. Or use the infinite MP and SP to upgrade skills.
Perhaps someone more knowledgeable, can share more information about the step by step to modify each value of the game. But this is my momentary solution.
I hope you find it useful. Greetings!