The heroes ARE needed (but you can chose your own) in this level since spells are blocked, preventing the best spell in the game from being used (Clarion Call).
The starting position I used was as follows:
Wave 3, use one hero to interrupt plague piper.
Wave 5, use one hero to interrupt plague piper.
Wave 7, normal fighting position, and also you can see which towers have been built so far.
In the beginning the top left poison tower should be focused on.
Around Wave 3 also the bottom left poison tower should be improved. (wave 3 is very hectic, some micro management of the heroes required).
Also increase the Tesla near the WoS to get range to attack the enemies at the fighting position.
Try to interrupt the plague pipers, they can otherwise create too many enemies for your few heroes to block, and one pesky bugger may slip through!
After wave 3 some enemies will every now and then pass over the bridge, they are not many, but another Tesla on the other side of the bridge helps here. You may also have to send a hero to help stop them from passing (None shall pass!).
In the end some more pictures showing where towers were placed.
With this setup only wave 3 was hectic, but some microing of heroes here and there required through out.