I had some problems as well with the game. (I don't really have any experience with this genre).I really dropped the ball on this game. Haven't even finished the SP. I'm at Avalon VI and dying constantly :/
But what I found very helpful was this list with the elemental weakness of the enemies:
http://soulsacrifice.wikia.com/wiki/Elements
and I started to farm missions to upgrade my spells. Here is a list of all mission rewards:
http://soulsacrifice.wikia.com/wiki/Quest_Rewards
16 of the same type upgrade to the next level with higher damage. Or for some you need do farm the individual parts and fuse them, if they are not available directly as a reward.
(I use mainly the weapon type spells and the damage increase is very noticeable.)
Armour spells are helpful as well, even for bosses that don't afflict elemental damage. Just to be a bit more robust.
I'm not 100% sure about this, but I read that most damage from bosses is non elemental damage and this counts as force and is reduced by the sigil force damper which you can get with Golbin LEs (+ and ++ for the higher one). If you're neutral the sigil is even stronger.