Good effort, man! I've beaten them a few times over the years.

I like to use All Lucky Sevens against them to start with. Make sure your HP is below 7777 then get into fights against weak creatures until the last two digits on one of your characters hits 77. Then you'll be on say 6077 HP or whatever. You can just use potions and hi-potions then to bring you up to 7777 then pick a fight with one of the big bastards

It won't kill them outright but it won't take much more of any of the above tactics to finish them off.