HL Boss: Jump up using the springy jump-boosters and climb up the platforms around the outside of the cavern. You're looking for one with a rock on it. Hide behind the rock and get used to where you need to stand (maybe crouch, I forget) to avoid his shots. On the walls around the boss are some large yellow nodes that regen his health. Take them out with rockets. Once they're all done, shoot him until his head opens up and then either try to loft a rocket/grenade inside or jump in there yourself to shoot his 'brain'.
Legend of Dragoon: Srsly, wtf, guys? I was disappointed at how easy this was, it was anticlimactic. He attacks like he's only just woken up and crawled out of bed. It's not hard to deal him enough damage that he transforms, and then transforms again because of the overflow. He doesn't hit that hard when he does remember to launch an attack, either. I can only assume you are missing a lot of the more decent equipment, which isn't something you can really fix at that stage. How the hell are you able to stay alive for an hour, but not deal him enough damage to kill him in that time? It took about twenty minutes on my first playthrough and less than ten every time after that.
Doom on Nightmare: There's more badguys. So what? It's not like you can't kill 90% of the stuff in the game with one or two shotgun blasts. Use the scenery to restrict the amount of things that can draw LOS and keep strafing. Boss fights aren't any more difficult on Nightmare, either.
Ruby Weapon: I killed this thing with no summons and no Final Attack. It took 4 hours. /smug mode. Load up one character with all your best stuff. Mystile armor, Ribbon, enough HP/MP boost materia to max out both, W-item, W-Magic, Added Effect + Time materia, Enemy Skills, Mime. And I guess Quadra-magic and Knights of the Round if you want to be a wuss about it. Ultima + MP Absorb can be handy when his tentacles are available as targets. Kill the other two characters off before you start. Begin by casting Haste on yourself and Slow on Ruby, since he gets four actions to your one otherwise. Then either just spam the summon on him and heal when you need to, job done, or BE A REAL MAN AND KILL HIM WITH YOUR SWORD. He's not immune to all status effect: Slow, Stop and Paralyse (from Needles item) all work, which is why you want Time paired with Added Effect. Start by spamming Magic Hammer from Enemy Skills to empty out his MP. This deprives him of several attacks and makes him stand there like a dick trying to use them and failing. Once that's done, drink a Hero Drink and start hitting him, using Omnislash where available and Miming it. Putting yourself into Frenzy helps build the Limit Break up fast. Heal with Elixirs or X-Potions, and if you need to heal twice in a row, Mime it so that you don't have to actually drink a potion to get the effect. He should never be in a position to knock you from 9999 HP down to 0 before you can heal, so you'll be in less danger than his massive hits might suggest. Just be careful of Slow/Haste wearing off, and the short reaction time that having his tentacles ready to attack can leave you with.
>You can also Mimic Quad-Cast Nights of the Round if you want (or Quad-Cut with Cloud) to beat Sephiroth really easily.
One single application of KotR will more than kill Sephiroth. He has 40k HP or so, it does 80k-100k. It kills everything that isn't Emerald or Ruby weapon.
Parasite Eve 2: How the heck do you run out of ammo when there are free bins of it lying around? Unless you're using something that gets crap milage out of ammo like the MP5 or M950, you shouldn't run out even with one weapon. Two weapons, you should have plenty. If you rescued Pierce, you should have access to free grenades. It's not like it's easy to waste ammo. ....you do know that you don't have to remove every single bit of the giant boss's body, right?