The easiest way to get Lucky 7's against Emerald is to have one character have 1 HP Plus materia leveled up enough so that character has 9999 HP (it is possible that Barret can get 9999 HP without one, but the chances are you don't) and any second materia- this can be Mime or KOTR depending on your strategy.

Then all you have to do is make sure that character is at full health when Emerald uses his Arie Tam Storm attack Arie Tam (is Materia backwards) and does 1111 HP damage for every materia you have so your character who has two will lose 2222 HP....sending them into Lucky 7 Rage.

Another popular way is Chocobuckle- run away from 2222 fights (isn't that hard I think) and then assuming your HP is 9999 you can enter Lucky 7's whenever you want. If your HP isn't max and you don't want to use HP plus materia (maybe you're doing a certain challenge that forbids you using support materia) then just run away from a few more battles.


Also since the amount of HP your characters can gain from each level isn't set in stone there is no materia combination that will guarantee you 7777 HP.