Final Fantasy X/Kilika Woods
It's important that you train a bit here. The next few bits of the game will be easier if you do train before carrying on. The first battle you'll find here will be predetermined — it's a quick introduction to Kimahri's special abilities, Lancet and his Ronso Rages (Overdrives). Simply use Lancet on the Ragora, and Kimahri will not only learn Seed Cannon but it will automatically fill his Overdrive gauge! Don't use it for the mean time, however, you'll want to save it in case you run into trouble, especially against the upcoming mini-boss, the Ochu. Instead, use Fire with Lulu (switch her in if needs be) and one more attack from any of your physical hitters to finish it off.
From where you start, head right and pick up the 2 Mana Spheres from the chest. Head left from the crossroads afterwards. Along here, you can follow the long path through the trees, or head down the narrower path. They both lead to the same place, but along the narrower one you can pick up a Scout, a useful weapon for Wakka, that has Icestrike and Sensor. As far as enemies go, you'll only be up against a few different ones:
- Ragoras are the ones you encountered earlier. They have two attacks, Seed Cannon, which does a small amount of damage to one person, and Seed Burst which confuses one person. Make sure you kill them with Lulu's Fire before they do, since Confusion is annoying, but can be cured with Yuna's Esuna white magic spell.
- Yellow Elements are the weird crosses of balls and symbols that float and move. Elements are highly resistant to physical attack, with Tidus only doing about 30-40 damage to it. They will absorb their element, and halve all other elemental magic except the one they're opposed to. Yellow Elements, for example, are Thunder element, so use Lulu's Water on them to kill them in one hit (it's basically the opposite of the predetermined Water Flan fight on Besaid).
- Dinonixes are greeny-turquoise lizards. They're "weak" to Ice but it still does little damage to them. They're quick too, which means only Tidus is fast enough to hit them, although Wakka doesn't have any trouble hitting them with his ball attack. Physical attacks are the way to go. Their physical attack sometimes (albeit rarely) inflicts Silence, or even Petrification (even rarer still). The only thing that can cure either at this stage of the game is Yuna's Esuna.
- Killer Bees are the last kind — they're aerial, so Wakka's the only one who can reliably hit them. They're weak to Ice too, which further makes Wakka's new Scout weapon useful, but they have magic resistance so you might want to avoid using Lulu. Their physical attack inflicts Poison most of the time.
You'll see the Ochu around — it's powerful. You can avoid it by following the path north from the crossroads to the left of it, but it's a good idea to fight it anyway (save first).
 Mini-boss: Lord Ochu
Of course, with just over 4600 HP that gives it more than twice the HP of Sin's Fin or Sinspawn Echuilles, and it's got powerful and more annoying attacks as well. Poison Claw does about 120 damage and inflicts Poison, which you should cure as soon as possible (using an Antidote from your inventory or Yuna's Esuna). It also has Water, which it can cast on you for just over 200 damage to one person, and can fall asleep and restore a LOT of health. Lord Ochu is immune to Dark Attack but he is susceptible to Silence Attack if you've learnt it, which rules out Water. He also has Earthquake which will do about 150-200 damage to everybody.
As for damaging it, you'll realise it's got too much health to beat normally and if it falls asleep it'll regain more health than you can take off. Use Silence Attack to begin with and generally just let everyone take a turn doing whatever so they all get AP at the end. Then, summon Valefor with Yuna. If you keep using Sonic Wings, although it deals only 100 damage a shot, the Ochu is slow enough that it won't actually get a turn (or it might get one or two — either way). If it falls asleep just attack it to raise your Overdrive, then use Energy Ray on it do a lot of damage — easily 1500+. Watch out for Water though if it becomes Unsilenced — this is pretty much one of the only attacks that will hit Valefor that often (Poison Claw is irrelevant since Valefor is immune to Poison, and every other status effect for that matter, besides Curse which doesn't show up until much later) and it'll do between 150-250 damage. Valefor has no means of healing itself either, so watch out.
You'll get an MP Sphere for winning, and you can talk to Luzzu afterwards for an Elixir. Before you continue any further, return to the Save Sphere and save, or at least use it to heal/revive Valefor and your party without using precious MP using Cure or Potions. Valefor will make the upcoming proper boss battle significantly easier, especially with an Overdrive handy. If you need to build it up again then summon Valefor in a random battle and have him use Boost (right on the D-Pad) to help it charge faster. It doesn't matter if he takes some damage; you'll only need one turn to use Energy Ray anyway, but make sure he doesn't die.
The commander lady at the third crossroad along the main path of the forest will give you a Hi-Potion. Take the north path here and follow it until you reach a chest containing a Luck Sphere. These are incredibly rare and not much use at all for the mean time, but you'll want to pick it up to save time later. The MP Sphere you got earlier creates an MP +40 node on the Sphere Grid in any nearby empty node which can then be activated by a normal Mana Sphere. The Luck Sphere creates a Luck +4 node which you can then activate with a Fortune Sphere. You'll notice at this point that you don't have any Fortune Spheres. No ordinary random battle in the game will drop any. Ignore this for the mean time, but don't use the Luck Sphere. You might want to wait before you use the MP Sphere as well.
Anyway, head right at either the second or third main crossroad and head north along this path until you get to the steps (the red blinking arrow will guide the way if you're lost). After the scene, save using the Save Sphere and head on up the rest of the stairs to Kilika Temple. What's that, Sinspawn? Oh dear…
 Boss Battle: Sinspawn Geneaux
If you have anything with Sensor in this battle, the information it gives you is pretty much enough to fight the battle anyway. The basic premise is that whilst Geneaux is inside its shell, it will be barely scratched by physical attacks and all magic gets absorbed by its tentacles. Sigh is the only major attack Geneaux will use in this form, which does about 100-120 damage to everyone. The Tentacles have 450 HP each and will do a weak physical attack that hits for about 90 damage. Valefor is useful since it should have enough health to survive a lot of attacks, and will dodge most of them anyway. Once Valefor has a full Overdrive, you can use it to inflict about 1000-1500 damage to Geneaux and instantly Overkill both the Tentacles.
Once both the Tentacles are dead Geneaux will emerge from his shell. His attacks now are Venom, which does 100 or so damage and inflicts Poison, Water, which is a bog-standard attack (that isn't as bad as Lord Ochu's Water) and Staccato which hits everyone for about 150 damage. Geneaux is weak to Fire so use that for good measure once both Tentacles are dead, otherwise just attack it (it no longer has the resistance to physical attack it had inside its shell) for about 200-220 damage. Fire will do around 500. Make sure everyone participates so they all get the AP. Head up the stairs once it's over and watch the scene, and you'll be outside Kilika Temple.