I dunno if I’d call it ‘painful’, but, well, I’m a Xenofreak. Anyway.
All of the characters are potentially useful at some point or another in the game. Though I found I wound up using KOS-MOS less than most everyone else, mostly because Jin can also ‘Air’ opponents, and he has the benefit of being ‘the new guy’. Shion, MOMO, and chaos should certainly have the Medica techs learnt. Don’t bother with the ether attack techs, they’re mostly useless. The vast majority of your damage output in this game will come from stocking, using Air or Down on an opponent, then boosting like mad to pummel it while it is defenseless. Keep in mind that Ziggy and chaos cannot attack an 'Air’ed opponent (or one that is otherwise flying), thus I tend to use Ziggy or chaos to inflict the ‘Down’ condition on enemies, so anyone can attack them.
There’s a lot of trial and error involved, unless you have a guide, since every Pc has some sort of ‘type’ on their generic attacks, and most enemies have at least one resistance (if an uncommon one). All of Ziggy’s attacks have a Fire attribute, all of Shion’s have Lightning, all of chaos’s have Spirit, etc etc. Once you know what an enemy or boss is weak against you can pull that character out and go nuts. In fact, one of the best places to level is in the Dammerung restricted Area where you just have Shion, and all the enemies there are weak vs Lightning. Any level she gains the others gain too, though they don’t get the points. (I outlined this in my FAQ, anyway. Even if you aren’t working on GS Campaign 32 it’s a handy thing to try.)
Neither of the 7 PCs are useful at every point of the game, you’ll have to do some switching according to the enemies you fight. Jr. is one of the most versitile in terms of attacks and weaknesses, being that there aren’t many enemies resistant to Piercing attacks, but also there aren’t many enemies WEAK to them either. He’s on the middle to low range when it comes to HP too, compared to the tanks, Ziggy, KOS-MOS and to a lesser extent Jin.
As for skills, make sure you snagged Psycho Pocket early on and give it to at least two characters, so that you always have someone on hand who can steal. This is primarily for stealing the four Awakening items, but it has its other uses. Aside from the Medicas, the Refreash L and Refreash H skills are important to have. THe status effects are grouped a bit oddly, there can be an L and H version of most. H’s are much more potent, but don’t tend to show until near the end of the game. L status effects are represented by a White-ish dot above the character, while H status effects are represented by a Yellow-ish dot. It doesn’t matter WHICH status effect it is, Refresh L and Refresh H will get rid of their respective ailments, but keep in mind they don’t always work. Though I’ve found the success rate to be around 80 percent, maybe 75. You also want Revert, which is basically your Life spell.
Giving the high HP characters the ‘Attacker’ skill, which raises ATK but lowers DEF, can be a handy move, so is giving your characters mostly oriented for healing the ‘Defender’ skill (which does the opposite), particularly MOMO. However, don’t understimate MOMO’s attack in this game, she has improved greatly since Episode I. Quite a few enemies are weak to her attacks, which all have an Ether attribute, and I believe her attack value is tied to her Ether ATK stat. During the final battle she was probably my heaviest hitter. Also, make sure to give Focus I (and later, Focus II) to your primary healers, MOMO and Shion certainly. You do spend a lot of time stocking, and the EP return Focus gives you is more than worth it. (Focus I is inside Segment Address 06, while you can’t get Focus II until you finish the game once and return to the final area in the UMN)
The Down and Break techs (lowers attributes of Bio and Gnosis, and Mech enemies) are really only situationally useful. Perhaps you might want to give them to one character, or spread them out, but I wouldn’t worry about always having them on hand. Personally, I never really used them. But they can come in handy sometimes.
Stock I and Boost I are also very handy to use on a spare turn, they’re almost always useful. Boost I especially, since the Boost Gauge is shared for the whole party. Also, you want to try and give Expansion Pack to just about everyone, despite its high-ish cost, the ability to equip an extra attribute is too handy to ignore.
Another useful tech is Inner Peace, even if you can’t get it until you complete GS Campaign 26, it raises your agility every time you stock. Since you spend a lot of time stocking in boss battles (and most battles in general) this attribute is almost twinkish in nature. With Inner Peace your characters will turn into dodge-meisters for most of the battle, only losing it once they finally stop stocking. It’s not just limited to dodging physical attacks either, oh no. It’s a pity you can’t do GS 26 until right before the final area, but once you can, get it right away.
The Veil and Sword techniques are mostly useful as well (though I personally found Ice Veil and Ice Sword weren’t worth the points). You certainly want to get Aura Veil, Aura Sword, and Fire Sword. Maybe Thunder Sword as well since it can come in handy fighting mech enemies. The final boss also uses Aura techs, so it’s especially handy to have Aura Veil there for the party’s protection.
As for all the other ‘equipable’ techs, use your own judgement, since it varies with play style. I’ve found that the ‘Coat’ techs weren’t universally helpful, but having some of them did help keep MOMO alive.
Another thing to always keep in mind is the use of Ether Combos. This is especially important for healing, since you can use Double Medica 2 as a more powerful Medica All. Ether Combos are used thusly: Both characters need to have the spell in question learned. In the case of Double Medica 2, two characters need to know Medica 2. When one character’s turn comes up, Boost the other, then go into the initial character’s magic menu, highlight Medica 2 and press Triangle, provided that everything was set up right, you should see Double Medica 2 available. Casting it essentially gives you the effect of Medica 2 on the entire party. Both casters lose the EP cost of Medica 2, and the effect of Double Medica 2 depends on who the intial caster is. So if you have MOMO starting the combo, with Ziggy boosted, the amount of HP healed will be as if MOMO casted it. Thus, it is useful to try and give the Medica skills to everyone by the end of the game. Due to Ziggy’s low Ether stat his Medica 2s won’t be as useful, but he can participate in a Combo for the same effect so long as the initial caster’s Ether attack stat is high.
Other useful Ether Combos are Double Medica (Medica x2) for early in the game, Thunder Wall, Ice Wall, Fire Wall and Aura Wall (Their respective Veil techs x2) which casts the inital veil tech on the whole party, Double Refresh L and Double Refresh H, for those pesky enemies which inflict mass status ailments, and Pocket Rare (Analyse and Psycho Pocket) for stealing Rare items.
Whew, that was spammy. That’s about all the general advice I can think of right now. If you need any help with specific bosses and areas, feel free to ask.