To get off font talk: Which of the three demo-classes is good for solo play? Sword Master (even steven), Assassin (rouges do it from behind?), or Cleric (Tank)? I'm guessing Sword Master but are the other two classes worth trying out?
Sword Masters are what I'd recommend unless you want to try something less straightforward.
Decent amount of damage per hit, stuns enemies fairly easily even late game thanks to the Lv3 charge attack, and has a block move for those times when you don't have a window to evade.
Clerics seem to do less damage per hit, but have high defense and a variety of combo finishers ranging from AOE attacks to rapid-fire blows. They can block too, and if you block an attack right before it hits, you'll counter instead - dealing damage while negating any to yourself.
Healing without potions is nice too.
Hammer Smiths have slow, powerful attacks and combo finishers that they can extend by spinning their drill-hammer as they slam it into an enemy. They're fairly slow and can't block, however.
Assassin's have the highest damage-per-second, but they can't break parts as easily or stun enemies to compensate. I can never launch enemies with them since my launching attacks tend to kill enemies before they get stunned.
Mages have a variety of elemental attacks in their combos ending with a powerful ranged-ish spell as a finisher (depending on which move you start branching from), but their regular blows do little damage. Holding O during a finisher will allow you to chant an incantation that powers up your spell if completed successfully. They also have three levels of charged attacks but the final levels don't hurt enemies at all and instead give you buffs.
Hunters are pretty counter intuitive, since their attacks have good range but do an amazing 1 point of damage per hit if you hit an enemy at max range. They do most of their damage close up (a sadly mediocre amount compared to the other classes) but make up for it with AOE attacks and rapidfire attacks that have decent stun.
Another support class.
I'd say that if you don't like Sword Warrior, then a Cleric, Hammersmith or Assassin would be a good choice for soloing missions.