Gaulen's poison is indeed very good. He needs poison, a primary weapon skill, and 2 of the exploration skills (herbs and terrains), which by itself takes more skill points than you get from levelling up. Stressing his skill point distribution even further by putting him in the front and forcing him to invest in durability is not the way you want to go with him.
Bleeding, wounds and stuns... all good, but it's not so good to have them all on the same character, since a character can only do 1 thing per turn. Specialists are the way to go for those effects. Thief for bleeding, mage for wounding, mage or barbarian for stunning. Arcane soldier also provides elemental damage over time, a niche that no other class fulfills competently (mage tries, but fails). Soldier feels like a crummy jack-of-all-trades type in comparison.
Soldier applies weapon mods at least 2x better than any other class (give or take equips, agility, monster resists and skill), it's his specialty - he is not a jack-of-all-trades in any way since that's really all he can do and he does it marvelously. A Barb with the top tier axe can do about 6 wounds per turn to a late-game HP sponge, a Soldier with that same axe can apply 12 or more. 40 wounds are sufficient to take any big, non-self-heal guy out of the game. One action per turn, twice the efficiency. Not only that, an axe Barb at start is an axe Barb for life. A Soldier can mix and match all 3 mods - extremely valuable, since a lot of monsters are resistant to 1 or 2 but not all 3 (good job balancing this, numantiangames!). It doesn't matter that a Barb crits twice as often when a mob has 10 000 HP and it's the -500 bleeding mod that will make the difference, not 50 vs. 100 dmg per hit. Again, a soldier can apply this bleeding mod twice as fast as anyone.
I'm not saying Barb is useless in comparison, far from it. He is simple, requires very little extra investment and can solo the arena at level 25. The soldier simply begs for every extra bit of +1 energy you can find so he can start using his skills. There are probably party builds and Barb builds that make him into that very cherished superclass, I don't know because I haven't tried.
I'm not saying anything at all about AS because I haven't played one.
With my party, however, it was the Soldier who shined the most from start to end.
As for Gaulen, I had no problem investing half of his skill points into learning and still having more than enough to invest into herbs and axes and misc stuff like poisons later on when he had the energy reserves to abuse it (not sure why you'd need to max exploration? I only started putting points to not die in the big desert and kept it at 10). He never felt weak at the front lines, simply because Axes are such an amazing weapon type. Then again, I'd probably try ranged Gaulen if I were to replay it, just to experiment.
Bottom line is, there are no 'useless classes' or 'useless builds' here despite all of them being interesting and distinct (well, not if you're trying to gimp your guys on purpose). Which is why I would encourage a beginner to experiment and take in that the glorious feeling of 'hey this works!' as his own. It's a big, fat plus in favor of Xulima as a crawler.
Good to see a divergence of opinions - a good RPG system should always incite debate! I started a party earlier with a thief, cleric, barbarian, bard, soldier and of course Gaulen. Is this a decent enough party to handle the game? Playing on the difficulty level above normal.
Yes.