Tentacles resist physical damage. They die to magic rather fast. They are good for farming early because they often alone and they spend half the fight trying to grope Celesphonia instead actually fighting...
I don't know if it's specifically magic damage, but I know that most monsters seem to be weak to 'light' damage.
Mutated humans, however, seem to resist it.
So my main build was just go buy/craft the light damage blade and invest in physical skills for multihit attacks.
The 'two random hits' is a workhorse move. Sometimes it wastes itself but it's pretty good at spreading the damage around.
Penetrating kick does great damage and cripples their defence, good for bosses, and the roundhouse (windmill?) is great for oneshotting whole groups of scrubs.
Also, it uses SP which regenerates, instead of MP, which doesn't. Meaning that you can save your MP for healing spells.
As you level up you get access to light magic spells without needing engrams too, which is pretty powerful.
The initial one is kinda weak and only hits one target, but it does light damage so it's good for oneshotting weak monsters early on.
But then you get access to a 'hit everything' mid-damage light spell for sweeping scrubs, and a single-target heavy damage spell for bosses.
The biggest advice for anybody playing is that you can often kill enemies on the overworld by attacking them from behind.
And you still get loot, exp and monster souls from them.
Get good at that and you can skip a lot of battles.
For bosses, i've never had much trouble. though I'm only playing on 'hard'.
Just go in with full health, cripple their stats with your physical moves and then beat on them. If your hp gets low, use magic to heal.
*shrug*
Oh, and upgrade your sword.
