Because it took me awhile to figure out a viable strategy I'll post this strategy so anyone else has a bit easier time. Certainly not the best approach but it works in 1.4.0.
Now my aim was to finish a map in hard mode with the two times exp leader and get some extra exp in bonus round.
I chose the second map because I like the big patch of green right at the start. I haven't got a picture so I'll just number the fields from left to right. The first row would be 1 to 4, the second row (touching the first 5 to 7, then comes the way and the third row 8 to 10. Then there is the lonely field in the water that grants one extra range. And then there are all the rest of the fields clumped together, the backdrop as I call it.
My skills are the first four damage full, the challenge damage full, the extra tower slot for yellow and below, time scepter (not strictly necessary, just convenience), 500 waves, the extra tower exp full and everything else in item drop chance. Oh, and I only unlocked the hydra arrow so I am pretty confident it works even without legendaries.
The first rather high hurdle is getting past wave 150. Or at least it was for me. The only way I found was to go for a wolfepack.
So nature is a must. For a second element I chose metropolis. If you are unsure, want to test the waters and so on, choose only nature. Taking all three will probably end in failure though.
Start by dropping either a baby rabbit or a dandelion on field 6 and something else in the backdrop. That place will be your main for quite a while. Let the dandelion soak up as much exp as possible. Recycle all towers to get wolves or items. Fill the main with the best stuff, the other tower is fine with the scepter of time. Recycle any potion to get either yellow potions or water of life. Again concentrate on your main. Drop your first wolf in the backdrop and replace the dandelion with the second wolf once no mass waves are displayed. Drop wolves in the backdrop and once your main is reliably killing fill fields 1 and 2 with wolves. You should have nothing but the pack on the map.
Once that pack gets going you can stop the immediate recycling. Probably a good idea to test the pack in 200 waves first until you got a feeling for it. If you test it choose only nature. On the lonely sea field comes a beaver and once manitou drops place him on field 4. Ganesha gets field 8 and a wolfskin cloak.
That strategy should carry you to wave 230 or so. After that the wolf pack won't cut it anymore. To go further start around wave 170 to basically a new strategy. Replace the wolf on field 2 with a Mr. Iron. Find time to get the frozen set and exchange it for the frost demon. Feed Mr. Iron with any item you got but no pictures of pretty girls. Over time he will become incredibly fast and deadly. Give him the full withered set and one wedding ring. The alpha wolf gets the other, the laser and droid, helm of Hades, Excalibur and a 7 mile boots (or hydra arrows in my case). The wolf on 1 gets the impatient set, that splash cleaver and some exp item. Your beaver gets any spare lucky pants and should be grateful for them.
Keep going until your wolf pack whimpers out. For the big switch replace wolf on 1 with the frost demon, the alpha wolf with scarface and all the other wolves with metropolis towers of your choice. Lots of Irish pubs mainly.
Keep going, feed Mr. Iron, skip seconds to hasten the frost demon and wait until you hit the bonus round. Congratulations. In my try Mr. Iron had about 12k extra damage and an attack speed of 0.05.
Replace Ganesha with the stonecutters temple. Remove the impatience set from the demon and put more 7 mile boots and / or two items from the frost set in it. Switch the helm of Hades to Mr. Iron, recycle nearly everything, drink all the potions, recycle the two wedding rings and give Mr. Iron an unstoppable force (i always park these things in some random tower so I won't recycle them accidentally) and scarface whatever you want.
What happens is that due to his insane speed Mr. Iron will teleport back creeps, add the extra dmg debuff and do a lot of damage. The frost demon will slow anything down, especially with the beaver stunning. Scarface will do broad damage. I got to 4200 bonus seconds with this strategy (and gained three levels in one run). The stonecutters temple will have a pretty big amount of membership levels and being level 99 the bonus damage will effect every creep.
A note for the potions. Once your two main towers hit around 1000% item chance you can probably start to recycle the water of lifes to more useful potions. Wait with all purple potions until you got both wedding rings and the towers are married (the second part is kinda important and easily forgotten). The tower on field 6 is the only one needing luck (Mr. Iron will get lots of it through Viking helmets) and any level up potions benefits mostly Mr. Iron (if he isn't level 99 already) and the wolf on field 1. That way the frost demon will slow done creeps further.
Anyway that's all folks. Have fun and good luck!
P.s: oh and Andy? I got the hydra arrow on two consecutive weekends... And when recycling that I got two items I already had... That was really annoying especially as i have only 8 golden towers, 4 golden potions and 17 golden items total. And four of the 17 are dark forge items. Which I don't have golden. Apart from that good game though.