i know i have an assload of game genie codes i used to use. the only one i can actually find now though is i think a gameshark code for secret of mana on the snes. change the value at memory address C040BE to BD, and that makes it so nothing takes damage. as far as i can tell, that applies to enemies too, so its kind of useless. but hey, lol, its a cheat code
E: this one isnt really a cheat code as much as it is an exploit, but in final fantasy 6, the evade stat does nothing. physical and magic blocking are both based on a stat called mblock, and if you pad that stat up to at least 128%, you will block almost everything in the game.
another fun one in ff6 is the vanish-doom exploit. doom is an insta kill magic that normally has a really low hit rate, and vanish is a status altering magic that makes it so you cannot hit an enemy with physical attacks, but magic has 100% accuracy. so essentially, ANYTHING you could use vanish on you could insta-kill, including like, 70% of the bosses.
E2: also, there are several game genie codes for zelda 2 that will swap the "shield" magic out for any other spell, but still use the same amount of magic as shield. so you can replace shield with thunder and be able to use thunder a lot more. thunder causes a lot of damage to every enemy on the screen, usually one-shotting them, but it costs a lot of magic to use. there is a side effect to this. the replaced thunder damage effect persists until you leave the area, whereas the normal thunder magic only damages everything on screen the one time you cast it. because of this, when you use that thunder spell on the thunder bird, instead of just making him vulnerable to sword damage like normal, it basically insta-kills him, but does so in this glitchy way that stops you from progressing to the next area. so moral of the story? replacing shield with thunder is awesome until you hit the thunder bird, lol.