The Lucky Sevens so called cheat is not an actual cheat, instead it is a special ability that all characters have and is extremely rare to get since it involves a lot of 7's, hence the name. There are many different combos in how to achieve this ultimate ability, but here is the simple way to do it:

REQUIREMENTS

There is just one requirement you need to meet to achieve this ability and that is to have your HP over 7777, so from 7778 to 9999 HP. If you are below that you will need to either train up hard or use the simple solution of having HP Plus materia equipped.

TRIGGER LUCKY SEVENS

Now that you have met that requirement here is how the ability will be triggered. When in any fight, enemies attack you of course, but if the enemy takes away enough damage to leave you exactly with 7777 HP, the character will go into a frantic fit and just launch themselves into the enemy hitting 7777 damage constantly without stopping, plus the damage is all non-elemental so only Barrier, Shield or Big Guard can reduce the damage slightly.

There is some bad news though, first thing is that it is just hard to get, secondly it only lasts for one battle and the most crucial disadvantage is that once the battle is over the characters HP will be reduced to a mere 1 HP so if you are in a sequence of battles that you can't cure between then this can kill off that character the next battle.