Those 25 strength points give you a damage bonus of 15. How is your offense/defense (ie chance to hit / be hit)? A high agility helps a lot.
differences between classes (secondary stat calculations)

If you approach slowly, you can draw off the lizards first, then go in and fight the dragon rider alone.

If you drink a shadow potion and use your spin attack near (but not directly on) a creature, you will hit it without turning visible.

Aura of Guarding or Spellshield would help a little, and mage spell books can be bought (check with Kistandalius or Blake in Ars Magicana and Corinna or Mpenzak in Verdistis).
If you quick save before running your mouse over an item/chest/merchant or hitting the alt key, you can identify the item or trade with the merchant, then quick load to get different stats or inventory. This can be done repeatedly to get the best stats or specific bonuses on an item (save after finding good candidates) or looking for specific items (such as spell books or charms) from merchants. Don't waste too much time with this, though, it really isn't necessary to get the best possible equipment.

As for the 1 hit kill, check your resistances. If they are negative (a bug with removing an item with a Magic Barrier skill bonus if you did not otherwise have any points in it), install the latest patch.

There are all kinds of ways to cheat;

iZakaroN's SaveEditor (experience, stat and skill points)

DAD's item editor

To copy an item; quick save the game, then drag an item out of your inventory, but not somewhere there is a red X. Without dropping first, quick load the game, then release the mouse button when it starts to load. This will produce a copy of the item on the ground, with the original still in your inventory.

There is a much easier method for items which can be stacked;
Drag a stack out of (or into) your inventory (or container) and hold down the control key before dropping. This will bring up the split stack pane; type in a negative number and hit enter (the checkmark will be greyed out). This can also be done in the trade window.

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