It's getting easy again even with all the difficulty modifiers. Here's my level 15 bloodwalker build for Sion, which has lowered the difficulty of this part of the game tremendously (around the end of chapter 2).
Final resistance in particular synergizes very well with bloodwalker. Will be giving him defensive choice level 17, just to make him even more tanky.
It'll get harder again as you meet new mob types. And wait till you try the Battle Mage sub-class, once you've ranked the sub-class up and gotten all its sub-class masteries it's like a super-tank - plus not
quite as much damage as Black Mage, but good enough.
The Common masteries and mastery sets are pretty good up to about your level, but you'll want to start filling in with some of the Elementalist and specialized Black Mage and Battle Mage masteries and sets soon (the Battle Mage sets are the most synergistic in the game, basically you just slot all Battle Mage specialties you get, plus some of the Elementalist, and you more or less can't go wrong, a whole slew of sets just automagically pop together, though lots of refinements in various directions are still possible). Irene also has a Battle Mage sub-class that works similarly, but is more based around roving/skirmishing than tanking for the team, which Sion eventually does superbly.
As the game progresses, you will find that you adjust your builds slowly as you go - e.g. Albus starts off as kind of your main tank, but he eventually becomes an amazing skirmisher/assassin in either of his sub-classes (though they play very differently).
The Lightning sp masteries and sets for Sion are great too (note what his main Lightning sp mastery does up top left and build in the Lightning masteries around that to start with), but they really only go with the Black Mage to any great extent, the Battle Mage doesn't have much use for them. Generally the sp masteries for all the characters (Lightning, Fire, Wind, etc.) seem unimportant at first, but they start to work their way into builds eventually.
Also note that once you've fully unlocked both sub-classes for a character, the abilities that were formerly specialized unlocks towards the end of levelling up the sub-classes become available to either class, and most builds mix and match abilities from both sub-classes in both builds.