A contest? Consider yourself screwed, I barely have time these days, and havn't played in a year. Can't even recognise your build right now.
Is that competition single player? Because in sinlge player you can farm interest much harder.
If you are playing multiplayer with all players on very easy, you should go for a build with slow and zealots, like dwfe (if that still exists, with zealot, muck, corrosion). You need to have pures in the endgame, I would strongly suggest not going pure dark, the other 3 are all ok. Play at 7 and or 8, kill left of 8. (Numpad notation if you don't know it yet.)
On single player, you can play at the junction between 5 and 8 (to get 2.5 times of shooting at the same wave), but let the creeps get to the right of 8 (thats right before the exit) before you kill them, to make more interest. Don't take interest picks though (or at most one). You should be able to get maximum gold (100000) if you are very good at this, just by covering the entire path with slowing towers. Like 6-9 each of super novae and monsoons will make the creeps so slow that you can make big cash. Just never overdo you towers, try to never kill a wave to early, even if you have the elements and the gold to build lv2 triples or so. Obvious builds are the double-slow builds, lwfn and dwne (if in fact they havn't been changed since 4.1 or so). Lategame the polar/voodoo combo is abuseable, as it does dmg based on a percentage of the creeps health, which makes their dmg grow with the creeps power. But that's a 6 ele build, and you can't max both towers. Even worse, you cannot max more than two of the key towers (which are the four slows, polar and voodoo).
The reason for this is that the developement team spent over a year redistributing the towers to prevent overpowered builds. We has ballance.
A single player 6 ele lategame would consist of the entire path beeing slowed by like 4x7 of the four slows. You would go insane on polar/voodoo, basically cover the entire map with em. And at 9 and the "-" area you would set up your dmg group, which will be periodics and any maxed short range dmg tripple you can come up with. Zealot would be ideal (played at 9, so they never loose speed). Stay away from drowning (works bad on high hp), anything with long range (like impulse) or flesh golem and gold (which are weakened to compensate for tricky abilities).
Your game won't focus to much on duals if you go for the 6-ele, as you can't max em out. But don't forget to add all dual support you have in your build close to your dmg towers, and if you go for a 4-ele, you really should max one of them.
Oh, and practice a year, that will help a lot.