Hiya, I've been playing with spells alot more with my current character, and my next will be more mage orientated.
I have about 150 magica, which I know is not a huge amount. I bought a Hail Storm spell which is the most destructive spell I can purchase and utilise right now. But it isn't that powerful and my Blade tends to finish the enemy off. My destruction is at 55 right now, I use fireballs mostly.

What are more damaging spells I can buy, or create? I'd like something which can take down a zombie in less than 5/6 casts.

I just created a mage and i dont know things much but,there is a Mage Guild member that sells Destruction spells in Skingrad.U should go and see him.

You have a series of 110 damage on target spells with fire, shock and frost you can buy but they are pretty expensive to cast. You also have area effect 75 in damage that is even more expensive.
http://www.uesp.net/wiki/Oblivion:Spells

I prefer self stacking spells myself; shock damage 50, weakness to shock 100 in 3 sec and weakness to magic 100 in 3 sec. make a similar with fire damage and weakness.

Here the fist hit does 50 in damage, the second 150 in damage, then 250, 350 and so on as long as they hit before the previous weakness times out. This spell kills almost everything in 3-4 hits; the gatekeeper requires perhaps 8. 50 in damage is a overkill as the damage adds up 25 in damage would just require a extra hit or two to kill most monsters.
Prefer to use ranged attack myself.

One other option is to have a separate weakness spell to cast first then damage effect.

I like that idea, I have been using Weakness spells and poisoned arrows. I'l try making a stacked spell tonight, just need to get some money!!!

Remember that as you use destruction spells (or any spells from a particular school of magic, for that matter) your magicka cost for casting that type of spell goes down, as your skill level increases. So keep practicing.

And, for zombies, use fire.

And Lightning for Daedra. <3

I find some of the most death per magicka cost spells are illusionary mind control spells.

If my character, while hidden, tosses a command creature spell into a nest of goblins to start a big fight, she can then buff her nails and leisurely poison her bow for the one wounded survivor. A great way to kill a goblin warlord or shamen is convincing one of their comrades to do it. Illusion makes it easy to start big messy brawls that result in a lot of killing.

Using mind control spells effectively takes some knowledge, but it can be extremely effective and efficient.

Yes, they are often scary effective, if you face a group of goblins including a warlord the smartest should be to control the warlord. the rest of the group would gang up on him, hurting him while he would reduce the group size.
For npc frenzy work almost just as well and is cheaper, however you need to hit most enemies in the group as the rest might attack you.
Against mages it's smart to combine it with silence as the will take more damage, you lose the firework but they are unable to heal and summon.