Marlboros: The easiest way to kill them is have Quistis use Degenerator. If you don't use her, then you can still beat them. Just Junction Confuse, Berserk, Sleep, and Slow/Pain to your ST-Def-Js. This will make Bad Breath pretty mild, and all you have to do afterwards is have someone use Treatment to get rid of anything else. All you have to do after that is beat on it for awhile. Casting Doomtrain/Meltdown will make the battle go quicker.

Ruby Dragon: As with the Marlboros, the easiest way is to use Degenerator. This works on every random enemy. If you're not using that, then Ruby Dragons are still pretty easy. Easier than Marlboros at least. Before you attempt one, make sure you have high Vitality and Spirit to protect yourself against its Breath attack (I forgot if this causes physical or magical damage, sorry). You can also kill one person in your party. This will make the Ruby Dragon "tame", and it will only use physical attacks, Fira, and Aero. No Breath attack to worry about. Doomtrain/Meltdown work well as always. Other than that make sure to have a good Str and Spd stat.

Bahamut: He is actually a bit challenging. The most important thing to do first is draw 300 Full-Lifes from him, if you don't have 300 already. You can take your time drawing, since he won't do much to you unless you attack him. Once you are done drawing, then casting Doomtrain would be a good idea. It also would be a good idea to put everyone in Aura status and have Squall and Irvine/Zell use their Limits. However, once Bahamut's HP gets low, he will use Mega Flare. This does a couple thousand damage (I think it did around 4000-5000 to me) and it will kill you if your HP isn't high. Once he uses it, immediately have someone cure your party. Selphie's Full-cure would be ideal.

I highly suggest using either Selphie or Quistis for this battle so they can be your support. Quistis should use Mighty Gaurd and White Wind and Selphie should use Full-cure and Wall. It is important to keep your HP high, because Mega Flare does quite a bit of damage.

Full-Life is the best for HP (Ultima is stronger, but it should be used on other things). Other than Bahamut, you can draw it from the Tonberry King if you have a high level. Regen Rings refine into 20 Full-Lifes too. For Strength, Ultima and Meteor are the best. You can get 100 Ultimas from 1 Dark Matter. Meteor can be drawn from Ruby Dragons. Meltdown is the best for Vitality, draw those from Gaylas around the Trabia region. You can also refine them from Mystery Fluids. Triple is one of the best for Magic, draw that from Odin and Cerberus. The next best method would be to refine 100 of them from 1 Three Stars (Squall's card gives you 3 Three Stars). Reflect is the best for Spirit (I think), that is pretty easy to get. Haste is the best for Speed, that can be drawn from Vysages. The only magic other than the ones above that might be hard to find is Aura. Buy 30 Power Wrists from the Esthar Pet Shop. 1 will refine into 10 Aura Stones, which each refine into 1 Aura.