Duel wielding weapons with magic damage bonuses / enchantments will help your spells (there is no need to invest in the duel wield skill, as the off-hand penalty is not considered for spell damage).
I don't know if anyone has done a DKS mage guide (there are a couple for D2:ED, but various changes to skills and balancing were made for DKS, which would make subtle and not so subtle differences).
Lancet's
Dragon Knight Saga FAQ/Guide has some comments on skills and stats.
There is some stat discussion in the topic
Anyone help a newb with Warrior build ... ???, but it is more focused on warriors and rangers.
The maps in the D2:ED
guide are mostly still valid for DKS (though all but one anti-dragon zones were removed from the flying fortresses).
I have not played a mage, but from what I've read your skills look good, though I don't know if you need both Fireball and Magic Blast. Firewall would help with close quarters fighting (as would level 1 of Evade), and Destruction and Mana Efficiency would help in general. The ghost summon can heal you (though not necessarily always when you want it to) and cast fireball at opponents.
Often if you are having trouble there are other areas you could explore first, or quests you could turn in, etc.
Do you have your creature yet? Playing a ranger I was able to get it on level 10, but there are a few higher level opponents on the way. The creature can be a help just as a distraction. I found it best using a mage head, but it was awhile before I was able to find one (level 16).
The mine you need to check out is west of Lovis' tower and then south (check the map, a straight line approach would probably not be the easiest way to get there).
West of the tower and north is the enchanter, if you have not found him yet, who you can get to upgrade enchantments, if you have the ingredients. When possible I added Life Line enchantments to new armour (and later Increased Mana, when 2 slot armour started showing up). For weapons, Increased Magical Damage should always be your first choice.
If you find or buy any equipment, you can have the enchanter remove the enchantments to get the formula if you don't already have it. I kept a list handy with the level of formula I had, so I could keep an eye out for loot or at merchants that had something I needed.
Mage is So Hard! WTHWhy would anyone play a Mage in this game