1 - Squishy Mage: Capra Demon, simply because you meet him so early in your growth, probably don't have stats for a good shield, and casting in those close quarters is deadly
Mages can use other equipment temporarily. Examples include wolf ring and/or knight armor. Poise tank the dogs and kill them after dodging capra jump attack.
Alternatively, most mages will use dexterity in some capacity to speed cast time. A single point in it allows you to rapier poke from behind a shield. Heater and rapier cost very little from the undead merchant (can buy them right after asylum if you run there). Even if you don't do that, simply wolf-ring trading the dogs after dodging the capra should work with pretty much any +5 weapon of your choice, and the fight is trivial after that.
3 - Squishy Ninja (dex build): assuming you went more for dodging and don't have a lot in the way of shields or armor to depend on, I'd say Firesage Demon or Manus.
Manus I could see, but firesage? Blocking the magic is questionable anyway, in fact blocking him in general is questionable. Just run around him unlocked like usual and attack during his excruciatingly slow recoveries.
Actually, I find Giant Ornstein much easier then Giant Smough, Smough takes out so much stamina if you block and his slam has a much bigger aoe then ornstein's.
Smough in either form absolutely, positively can not handle getting around pillars. He'll shovel attack right into them, exposing himself to basically anything you want to use (Zwei 2h, halberd, spear, magic, pyromancy, bow, xbow, take your pick). Super O can stab you through the pillar, do his warping jump attack, and actually shoot at you. Smough can...continue to charge into the pillar, or jump into it. Or swing into it hitting nothing.
Super O can actually hit me! You can dodge most of his non-butt slam attacks by keeping close, but occasionally he'll do this back-jumping swing in front of him and it's too fast to do much about it if you're already swinging, meaning you either turtlepoke that or you can't attack him unless he's committed to an attack already. His stabbing, butt slam, and jump dash-forward are all threatening, and I have been stabbed by his "stab grab" THROUGH the freaking pillars before...on a base vitality build that absolutely will 1-shot you unless you are wearing lightning-resist gear. Super S has nothing on super O.
Overall O&S are probably the hardest unless your damage sucks simply because you still need to memorize patterns, but there are two of them. While manus and kalameet are no joke, they have patterns/tells and only require focusing on one guy.