Most phantoms, ghost and other spectres are weak to Holy, although other elements can usually damage them to some extent as well.
As an Assassin myself with almost completely useless pawn when it to comes to attacking enemies, I rely on her Hole buffs and consumables. There are items, books mostly, that enable you to cast spells for some damage. I always carry few of those with me.
If you want to go a Hybrid, I strongly suggest Magick Archer. It's the ultimate class imho.
Magick Bow will take care of ranged attacks and its skills are pretty strong and useful. Daggers will help with evasions and will give you the best DPS move, Hundred Kisses. Staff will help with traveling and can use some basic spells, but other than traveling, Magick Bow will outdamage it.
MK is very good, but it relies on buffs and parrying too much. It's also pretty useless when you need to attack something ranged, especially flying, because it's Great Cannon has horrible vertical tracking. Even though MK has a big shield, it can't block very well. Fighter is 100 times better for blocking and since the class lacks evasion skills of daggers, it will get hit a lot.
The game is generally super easy, so most of these don't matter, but once you step into post game Everfall and later BBI (if you get the DLC) then these will matter a lot.