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This really isn't that hard. First, obviously, run from all non-scripted non-boss battles. Duh. Boss battles provide 0 EXP, and so the only thing you have to worry about is not gaining EXP from scripted non-boss fights. For those, either make sure that Squall is KO, use the Card ability, or use Break. Oh, and the same applies to Laguna during the dream sequences, since any EXP Laguna gains goes to Squall.

Once you get to Dollet and Seifer is in your party, KO Squall and Zell, and put all 3 GFs you have at that point (which should be Quezacotl, Shiva, and Ifrit) onto him and grind AP). Make sure you at least learn the Card ability for Quezacotl, since carding enemies is a great way to gain AP without gaining EXP. Now, the Steam version of FF8 has a 'magic booster'. Use it. Also, you can get some pretty decent items in Chocobo World that you can transfer over to your FF8 main game save. For instance, you get an item that teaches GFs the 'Ribbon' ability).

Moreover, if you take the time to complete the Queen of Cards and CC Club side quests, you can card mod rare cards for decent items/spells and then win said cards back from the CC Club on board the Ragnarok in Disc 4. In order to get the components to summon Doom Train: 1) you can get Steel Pipes from Wendigos outside the forest right before you first get to Galbadia Garden, 2) you can get Marlboro Tentacles by card modding Marlboro cards, and 3) once you get Alexander, you can learn the Med Data and Med Lv Up abilities by waiting until Edea joins your party, giving her all the GFs, KOing the other characters, and then grinding. You can also get Tonberry without getting Odin, since the Tonberry count doesn't reset when you leave Centra Ruins, and Tonberrys drop 0 EXP.

You also don't need to fight Ultima Weapon to get Eden. You can draw Eden from Tiamat in Ultimecia's castle instead. The only tricky fight can be the fight against Ultimecia at the end. There are certain enemies you can farm draws from... or just card them and use items they drop to mod into better magic. I just grinded w/Seifer in Dollet, and learned Med Data and Med Lv Up using Edea and then just fast-tracked to the end. It only took me two attempts to beat Ultimecia (on account of the fact that Squall was absorbed into time before I could revive him, and it kind of went downhill from there). Oh, and make sure to get at least 20 Elnoyle cards from Cid. You can card mod them into Pulse Ammo.