Auto-attack as a system itself isn't that hard to implement but then with the addition of auto-attack you have to alter and check:

1) monster behavior
2) monster stats (HP, MP, core states, sub stats)
3) monster abilities
4) monster spells
5) monster dps
6) player abilities
7) player stats (HP, MP, core stats, sub stats)
8) player spells
9) player dps
10) group play functioning with auto-attack
11) solo play functioning with auto-attack

I'm sure there are more testing/re calibrating they do as I am not physically there in their studios so I cannot give a complete list.