Charm Ammo in Shin Megami Tensei 1. Basic ammo for your guns, and you would be surprised what is not immune to charm in this game including bosses.

Swap spell and Unarmed in FFII, you want to break this game at the beginning? Drop all weapons and just have the party use unarmed, their strength will sky rocket and the unarmed counts as dual-wielding so the unarmed proficiency will max out to the soft cap fairly quickly. Now level up some healing spells, head to Fynn and fight those Imperial Soldiers walking about. They have one of the highest enemy ranks in and with a high enough unarmed level, you can actually take these guys out and watch a lot of your stats skyrocket. These guys also drop mid-tier gear and weapons that can boost your team and sell for a lot of money. Once you have 24,000 gil, play long enough to get the canoe and head southwest to Mysidia. You will likely need to run from everything since this is an end game area but once there, buy three Swap spell scrolls and teach them to Firion, Marie, and Gus. Return to Altair and wander about until you fight a simple Goblin, use Swap with one character on the enemy to trade your HP/MP and finish the battle. The Goblin's stats are so low, you're guaranteed to see your HP and MP skyrocket as well. Keep doing this with each character until their health is in the thousands. Now go beat the game and be completely bored as nothing can touch your team. Also the Toad spell in this game works like an instant death spell, what most don't know is that several bosses don;t even have immunity towards it, so if you really need to level up one offensive spell...

FFVII, once you get out of Midgar, give a character with the highest HP (usually Cloud or Barret) a Fire Materia linked to the Element Support materia along with the Enemy Skill materia. Place everyone in the back row and have one other party member with the Poison Materia. Fight the Midgar Zolom instead of using the chocobo. Bio will Poison the sucker at which point you just need everyone to stay healing each other. Once it rises, continue to skip the turn of the character equipped with the first set-up so they'll be able to react immediately. Once the enemy reaches a quarter health, they'll cast Beta. The character with the Fire/Element combo should survive the attack and since their turn should still be available, have them cast Beta to end the fight and then save. Beta pretty much makes everything in the first disc a complete joke and no, you don't actually need high levels to do this strategy, maybe an extra restore materia but its actually pretty easy to pull off.

In Suikoden I, you can recruit a guy to play a game of dice, he is incredibly bad at it thanks to faulty RNG and so you can amass unlimited money to keep your whole army completely updated with the best gear and highest weapon upgrades available.

In Suikoden II, once Riou has access to the Matilda border, you can actually push on the gate despite it being locked and move the door open to get past it. This drops you in a high level area but if you can make it to the town, you can recruit Humphrey and Futch who will be scaled for this area and then power level your team to absurdness. making the first half of the game significantly easy.

In Suikoden V, Ernst is a character you can recruit who has been cursed by his beast rune to take the form of a cheetah. His actual rune in battle transforms him back to a human in order to gain access to the rest of the rune's abilities. One of those abilities is a move that stuns all enemies on the field and Ernst is one of the game's fastest characters so he almost always goes first. What makes this truly game-breaking is that even bosses are not immune to this rune spell including the final boss. You can even gain a formation eventually that has the same effect though it only works once, but you can then use that turn to transform Ernst to spam his ability every turn afterwards effectively meaning nothing in the game can ever touch you.

In Lufia 2, once you find the island the Ancient Cave is on, just keeping playing it and try to amass weapons from the Blue Treasure boxes. Not only will these treasures help you get further into the dungeon, they are often endgame level gear as well.

In BoFII, when you get the chance to make the town, one of the carpenter's you can choose will open up a kitchen with his wife, where they'll take a bunch of easy to acquire items like some of the fish and meat you can hunt for and turn them into permanent stat increasing items, and one item than can be sold for more money than the price of all the easy to obtain items to make it.

In BoFV, the SOL system allows you a few options including one that send you back to your last save but sacrificing the personal XP you got but letting you keep half the Party Experience you acquired. Beating battles quickly tends to net you higher Party XP and doing this restore option also rests the D-Counter meaning that if you reach certain bosses close to a save point, you can cheese this boss over and over to amass millions of XP to power level your party to end game levels. In fact the second to last boss nets you a whopping 40,000XP if you can kill him and his two clones in one turn, which is ridiculously easy with the Dragon Form in this game. If you amass around 2 million Party XP farming this fight, you can make NG+ into a perfect run game by leveling your whole party to level 60 once you recruit them.

In Wild Arms, if you really just don;t want to bother with challenge, there is a ridiculously easy item duplication trick in this game, and unlike say... FFVII's version, you can actually use it on stat improving items. So you can basically have three characters with max health, Rudy will have infinite ammo basically for his ARMS, Jack can have all his Quick Draw abilities cost 1MP, Cecilia can have every spell in the game, and you can have 99 Escape Goats which auto-revive a character in battle when killed. All it requires is two basic Heal Berries (think the lowly potion from FF) and one of each of those special items special items.