Secrets and Hints
Here is a list of useful tidbits and fun things to do in Final Fantasy 7.
Getting back into Midgar
Here is a tip that I thought of myself, although a few days after I used it I saw it posted on another site. But anyways, regarding materia combinations: have you ever put an "added affect" materia linked to the Hades summon? With it, whenever you deal damage you might poision, shrink, mini, berserk, confuse or sleep the enemy... or all of the above! While this might be risky in some battle situations where a boss might drain your attack, it is REALLY helpful when used in the Gold Saucer Battle Arena in conjunction with the "Slash All" materia... imagine shrinking, poisioning and sleeping all of the enemies in one round with one hit! Instant BP.
I would suggest you at least defeat the Ultimate WEAPON, because it allows you access to one of the most helpful materia in the game: the Slash All! This is probably the most helpful materia in the game, excluding all those really cool Summons and stuff, because you can quickly clean up smaller enemies and deal a fair amount of damage to all of the tougher ones. It is also helpful when bosses have more than one Hit location, Like Jenova and Sephiroth. Finally, it is the best Materia to have in the Battle Arena next to Cure(regen) and Quadra-Bahamut ZERO.
In the 3rd CD, go back to the Shinra Masnion and down into Sephiroth's Library. You will find a special Cinema that clears up a lot about Cloud and his Party.
Also, another helpful place to go is the deserted house in Icicle Inn. Check the machinery at the upper left hand side of the house. Look hard, because the cut-scene you see there clears up more about the storyline than the one in the previous tip.
Once you get the submarine, explore the ocean thoroughly for a plane that has crashed in the ocean. Inside you will find some difficult enemies that are useful for leveling up materia, and some useful items. Among the Items are the "Double Cut", which can be deadly in a powerful member's hands, and the Hades summon (see first tip here). Also, Yuffie's Ultimate Weapon lies in the engine room, which is the only reason some people use the otherwise useless-and-annoying ninja: it can deal a whopping 9,999 points of morphing damage, enough to turn your least-favorite monster into your favorite item. Beware, however, that sometimes Emerald Weapon lies just in front of the plane, and can put a quick end to your treasure hunting.
Getting back into Midgar: Head to Bone Village and ask the excavator to search for "Normal Treasure". Place everyone you can at the top area, and then use your blast. With a little luck, the men will dig up the key to the sector 7 slums. Now you can re-enter Midgar.
While in Midgar, head to the Wall Market. Go to the guy who sold you the batteries, and see what he has for sale. Buy the "sneak glove", which is a very potent accesory that increases the success rate of the "Steal" materia.
All Lucky 7's: If during a battle you are dropped to 7777 hitpoints, your character will go into a rage, and deal a ton of damage to the enemies at 7777 increments. After the battle, the character's HP is dropped to one. Tips for getting 7777 HP: While very hard to do, it is possible to manipulate. Remember that High Potions restore exactly 500 points, and Potions restore 100. This means if you are ever at XX77 points and your max HP is above 7777, you can just use High potions untill about 7377 or so, and then regular potions to start the next battle in Lucky 7's. Good luck!
The Guidebook: In the 3rd disc, go back to the underwater reacter. Run as far as you can until you fight what looks like a ghost ship. Instead of killing him, use the morph materia (Yuffie's Conformer is great for this) and he will turn into a Guidebook. Give this to the Kalm traveler and he will give you the underwater materia. This materia is a neccessity if you plan on fighting the Emerald Weapon, as it removes the 20 minute timer.
That seems like all the tips that are really important... the rest are kinda useless, but if I missed one that you want to see here than E-mail me with your request.