This is a very loose guide since there are no other guides up here. I highly suggest playing in endless mode and leveling yourself all the way to 100. Get some good gear, if you are willing to put a little cash into the game then get the items you can buy with crystals. Otherwise just farm gear in endless mode. Even if you suck at endless mode keep it up. The exp there is much better than anywhere else in the game. You might only be able to go up to wave 4 or 5 when starting out. Maybe less.
As far as doing these missions. I was doing it with a sorc, but it should be easy with any class. By the time you get here and are level 100 you will do far more damage than your traps. So make a lot of traps that have the ability to slow down the enemy and such as mystic ice traps, both tower and floor and the base mechanical tower that has a slow as well. Buy everything that you can to slow down the enemies coming out of the gates and then run around and kill them all. As you gain more money then start upgrading the damage abilities of your traps.
Also use the essence trap. Put it where ever you plan to spend the most of your time fighting so you can get as much extra cash as you can.
The only stage that proved to be really difficult was the final stage. I suggest getting all the money generators in the first wave. Try stalling the NPC's as much as you can so you get more time to generate money.
There is a trick you can use if you are the sorc. Take the skill Chaos Armor and have extremely high resists of at least of around 1k or more for physical damage. When you get to the last group of 4 enemies kill all but one of the purple ones and just stand in front of it. The chaos armor should pop frequently enough that you will heal up all the damage you are taking from being beat on. I was able to save up 6k essence just letting the generators build money while the NPC was beating on me. After you hit 6k just kill the last npc and you have enough money to build all the traps on the map. The stage is extremely simple after this.
*WARNING* The game seems to detect that chaos armor is popping too much and it will stop working for a small period. You will need to move your character away from the NPC and then back into melee range with it again to get it to start working again. Keep healing potions on hand for when this happens.
I have no gotten to play the other two classes yet. Once I do if I see another trick that would work for them I will post it here.
You also can play this in coop with some friends or random people in order to make it a lot easier.
If you need more help feel free to post here and I'll try to answer as best I can. If you don't like this guide then write a new one instead of down voting because you can't figure this out. This is here to help people more than a blank guide.