First let me say that this challenge is pretty hardcore.
You need to be fast and precise when placing the reinforcements and the Rain of Fire. Beginners won't be able to do it. You need some experience in order to succeed. 
Having only magic towers and the Sorcerer upgrade means sooner or later you'll have to upgrade the Elemental. The Demon Hounds are fast and deadly. But what's really gonna give you the advantage is the other magic spell of the Sorcerer tower - namely the Polymorph... It transforms the unit into a sheep. This sheep is weak and relatively easy to kill, however most of the times I'd suggest killing them with the mouse. How do you do that? Just keep clicking on the running sheep and eventually it will explode. (the Ovinophobia achievement... )
Having many magic towers won't help you here. They do some damage, however it's simply not enough... Even if you manage to hold the first waves, the moment the Hounds start appearing you're doomed.
Thus you'll need to be on the offensive... Normally when playing this level one concentrates the most powerful towers around the T-Point closest to the exit. In this case however we'll need to move to the other T-Point on the map.
In the beginning you're lacking firepower. You need gold and that's why killing the opponents faster will make the difference.
Here's my starting tower order:
As you see you start upgrading directly from the T-Point. That's the place where you'll concentrate your efforts for now. Later on you'll have to move backwards and defend the other T-Point on the map.
Maximizing the efficiency of the Rain of Fire is priority. That's why I'd suggest using it 3-4 secs. after the start to kill the incoming husks from the right down corner. This will give you some time to deal with the Bats and upgrade one of the near towers.
Try to put the reinforcements directly on the T-Point, so they can deal most damage.
The next thing you'll focus on is upgrading the towers guarding the T-Point. Soon after that you'll be overpowered by the incoming waves and you'll have to back off for a while. When there's gold availabel try upgrading the towers closest to T-Points. Upgrade all the relevant towers to level 2! Don't jump to level 3 upgrades! You're lacking power now...
Gradually upgrade all the towers in the middle to level 2.
Use the Rain of Fire to kill the incoming monsters and clear the T-Point. Place the reinforcements just behind the are, so they can encounter the next enemies.
Upgrade the upper towers so they can handle the incoming monsters from the upper left corner.
By the time the Bats appear you should have enough money for the first Sorcerer upgrade:
Use the Rain of Fire to kill the first incoming Hounds. This will give you time to regroup and summon the first Elemental.
From then on you'll try to hold on till you have enough money for the Polymorph upgrade. You need luck and quick fingers, but it's far from impossible.
Remember! Don't let an Elemental get killed. At least not in the beginning. Towards the end when you have Polymorph it's not that curcial. I'd suggest spawning the second Elemental from the tower, that's just above the two in the middle. This way you'd be able to move him from left to right. Saving his life... Reinforcements are to be placed on the T-Point, that's closer to the exit so they can handle the incoming Hounds. The Elemental from the near tower shoudl help them out!
Upgrade the remaining towers to Sorcerer!
Fighting the Swamp Things (the big, green monsters) is the most difficult to do. You'll need to manually place the Elemental next to the monster so he can engage it.Reinforcements should be attacking from behind... Gradually start upgrading the Polymorph. 1 Level / tower! Better more upgraded towers with lower level Polymorph than a few ones with higher level magic.
Another hint... When fighting the Swamp Things... Observe carefully the health of the Elementals. If it drops make the next upgrade so the Elemental gains full health! VERY IMPORTANT!
Clicking on the sheep is also a MUST! Here you really need luck. Sometimes a monster is turned into a sheep and its speed is really crazy. You simply can't click on it fast enough. It has happened to me many times. Don't get angry! It's just bad luck!
And another win:
As you see the number of towers can vary. Basically the more, the better! But you must be able to hold the incoming enemies...
Maybe I'll post some more pictures, but for now I think that's enough!
I'm open to suggestions, criticism and praise!
Good luck and "To arms!"
P.S. Most importnat things:
- Build 2 Elemental: 1 guarding the T-Point next to he exit and another from the middle tower, so it can move to the right and to the left. Thus saving its life.
- You MUST change the waypoints of the Elementals! This, combined with the Polymorph will gain you victory!
-= My best result so far=-
