Just a little guide to save you some time backtracking.
Guide to Trigger the Good Ending
Collect the Power of the 4 Sacred Beasts
If you want to see the normal ending first then just face the boss without collecting the powers. Don't worry, after the credits you can load up the game again and you'll be right at the last spawn point before the final boss.
In order to get the 'Good' Ending and receive the 'Perfect' Achievement, you need to collect the power of the 4 Sacred Beasts before facing Chaos. If you are like me and forgot where the exact locations are, then this little guide should come in handy and save you some time =)
The four locations are as follows:
After collecting all of the above. Simply go to Chaos and trigger the final conversation (which will differ from the normal ending) and you should receive the achievement. Not even need to beat Chaos (again).