Note: most skills will have a larger increase in effectiveness at their 1st, 5th and 10th skillpoint e.g. HP UP increases the HP by 10%, 6% and 7% at those points, whereas it only increases by 1% at other times. Therefore most of yours skills, unless you only take them as prerequisites for other skills, should be raised to those levels before taking a new skill.
General:
- TP UP is the best skill. The 50% increase in TP means that excursions into the labyrinth can last 50% longer than without. Almost everybody should have this at 10. It's not something that you take early on, but later it is vital for actually achieving shit.
- ATK up is another great skill, for obvious reasons. Every class that has it should raise it to 10. Even the medics. Especially the medics.
- Any other Up skill is fine at 5 points. That includes the P's Def Up and HP Up skills, for reasons that I'll explain later. The only exception would be an A's element up for an element that they don't use.
- Every class has one gathering skill (except for S, which has all of them). It's better to have one character specialize in one kind rather than having several characters going only halfway, because, even though the amount of stuff that you can gather is the same, a specialist is just better at it. The other guy should focus on fighting instead. In general, unless it's a specialized S, don't bother taking more than one point of each.
- Scavenge is a useful skill for farming rare items. I suggest giving a few points of it to an A, since they're more likely to kill stuff than, say, a M.
Landsknecht: Those guys seem like they have two viable builds: axes and swords. And, ya well, yeah, can't use both at once, right? Actually, since axe skills are good at single target damage, while swords excel at groups, using both depending on the enemies you fight is a good idea.
- As axe skills go, Crusher is the only one I consider using. Stunner is obsolete with a M, and Silencer doesn't work on the bosses where it'd be actually useful afaik.
- Of the sword skills, Tornado would seem better than Allslash, but the latter can hit more enemies at higher levels. Cleaver is useless when you have Crush.
- I'm not sure what to think of the elemental attacks. Exploiting enemy weaknesses seems like a good idea, but when you have a T and Crush... I don't know.
- I'm not a fan of skills that increase one stat at the cost of another. Especially when it's the def of a frontliner. Not going to take the Cry skills, personally.
Survivalist: There's two general builds for those guys. Hunters and gatherers. That means damage dealers and people who specialize on gathering resources for money.
- They have some pretty neat stuff for combat. Ambush is a must have and makes Aware obsolete, afaik. Quicken is better than 1st Hit. Trickery is useful. Apollon is murder.
- Gatherers should focus on chop first, for reasons that I won't spoil.
Protector: Not much to say here, in general.
- Defenderrrrrr. Get it up to 5 ASAP. It will literally cut down the physical damage the group gets to tolerable levels. Later on it can go up to 10, but it might not be necessary. This is also why HP Up and Def Up aren't so important that you'd need to put them all the way to 10.
- Provoke... is not a good idea. It's unreliable. If you want to protect a weakened character, use the Guard skills that you should have due to Defender.
- The anti-element skills go up to 5, no further. The reason, as explained before, is that at 5 points it also grants immunity to status effects that elemental skills may carry. If you start absorbing the elemental damage, said immunity goes away.
- Cure, En Guarde, Parry, Aegis, they aren't that useful, and some of them are random, to boot. You're better off using Defender and Immunize to reduce the damage and your M is better at healing anyways.
Dark Hunter: Again, two possible builds, whips and swords.
- Swords are superior to whips in every way. Some bosses are immune to binds, and it takes too long to set them up to make Ecstasy useful against random encounters. Climax is more useful, but random encounters would die anyway and bosses are immune to the instakill.
- I haven't seen anybody use any active skill that isn't Drain or Bait. And for good reason, those skills are simply amazing. Boost + Bait = "WTF did you just do?"
- I don't like passives that only activate once a character is hurt. However, Fury activates at 50% damage already, which is pretty decent, so you either take it entirely or not at all.
Medic: Your healer. They can do more, but more on that later.
- IMMUNIZE! Best skill ever. This, combined with Defender and the anti-elemental skills, mean that your M will never have too much to worry about unless you're underleveled. Get this ASAP.
- CureIII sucks. You should be able to fully heal your guys with regular heals, so you shouldn't waste your SP on this.
- Similarly, CureII and SalveII will be prone to overhealing if you play your cards right, which wastes your TP. Salve should be more than enough to keep your party alive.
- If you take revive, don't put more than one point in it. The extra HP won't help them survive any longer.
- Unbind doesn't do anything on 2 and 4 points. In fact 3 is the most you'll likely ever need.
- CPR is not so good. If your guys go down to 1 HP, then they're dead anyway.
- H.Touch. Don't. Just don't.
- Regen is no good. If somebody's hurt, you'll want them healed immediately, not over time.
- Patch Up isn't affected by Healer. That makes its effect negligible.
- Caduceus is a great late game skill for when you have all the healing you need. Bonk!
Alchemist: I heard that poison is great at the beginning of the labyrinth. I personally never use gradual grinding, so I don't know if it's true, but if so one can first skill poison and later rest to spend the points on more direct damage skills. And of course, it's pointless to try and get all the elemental spells. You can get about half of the secondary single-target and/or multi-target skills, so choose wisely.
- There's... not much I can say about the elemental spells without going into spoilers about boss weaknesses. Let's just say that at least one of the elements will be a frequent weakness and should therefore have its second single-target skill put up to 10. Having another element's multi-target is good for grinding and hitting different weaknesses.
- Sight and Warp are both useful, however their infrequent usage means that you shouldn't put more than one point in each.
Troubadour: SPOONS!
- Divinity is great for leveling. You'll have to rest your T in the endgame to put the skill points into skills that don't become obsolete, but it's well worth grinding those 10 levels with a single character.
- Ifrit, Ymir and Taranis boost your party's resistances (useless with a protector) and lower the enemies'. I don't know if the second effect makes them worth the points, though.
- Since everybody who has it should eventually have Atk Up 10, Bravery is a no-brainer.
- Stamina's good too. Shelter doesn't quite compare to it if you have Defender and Immunize. And if you need a speed boost you take a S instead. Also there's a limit to how many buffs you can have, I think?
- Recovery is good if your M didn't take Refresh. Since this only takes one turn to cast on the entire party, and affects all status effects even at a lower level, I think putting this up to 5 and saving the M's SP is worth it.
- Erasure, like Unbind, gets twice the effect on 3, whereas 2 and 4 are lame. And like Unbind, 3 is enough for most situations.
- Healing has the same problem that Regen has. Also, you already have a M.
- Relax. Take it. More TP is always good, since it's an important resource.
- Blaze, Frost, and Shock are why I'm so hesitant about the L's elemental skills. Putting a point in each means Crushing enemies with their elemental weaknesses. This is what makes a T a better choice as a 5th character than a S for bosses.