You'll need to use those "other" schools of magic (Alteration, Mysticism, Illusion, Conjuration...)
For personal protection, Elemental Shielding (especially Fire Shield, early on, and most especially if you're doing the MQ), and Spell Absorption, if/when you can get it. Those fire towers in the Deadlands are deadly to your character, and scamps are more than a nuisance (they use Fire, and they can Reflect your spells back at you.)
Get yourself help (and Meat Shields) in battle by Summoning and by Commanding. If the enemy is fighting your Zombie, or among themselves, they're not shooting at you. For fighting Necromancers and Conjurers, "Silence is golden."
And never, never cast a heavy-duty Destruction spell at a Lich.
Another thing that helps is to use your Alchemy. Player-brewed Restore Health potions are actually valuable for this kind of character, because they restore over time. The idea is to drink them before your health drops, and use the Instant type only when you need to quickly restore. And make it a habit to keep track of how many active potions you have going, because you don't want to discover that you've got four "buffing potions" counting down when you suddenly need to restore your health.
It's really all about planning. There's a lot more micromanaging with a weak character like this. It can work well, though; my Silvonwe made it to level 20, and was Archmage.