This build makes use of Flamethrowers, Snaking Sands and a Poison Dart Tower Tower to deal with everything while the three Sky Guards are placed close enough to centralize on a few spots while clearing their corresponding lanes of air units.
With the exception of the Western zone, each flamethrower is built on a ledge or object. The flamethrowers up North are on the fences, poles and tree trunk while the flamethrowers in the South-East are on a deck, barrel and ledge. Ensure the blockade in the Northern-most lane is as close to the left post as possible without creating an opening to the right; this is to prevent zerkers from walking over the wall. If you can climb your blockade by walking into the gap, it's not close enough.
If you don't have snaking sands, remove one of the flamethrowers along the fence in the North-Eastern quadrant and place Frostbite Towers in each lane except the South-Eastern one.
In the second wave, give Sky Guards 1 - 3 upgrades, upgrade any blockades having trouble and use the rest on towers. Max your Sky Guards within the first three waves.
Flamethrower - Destruction, Empowered Flame, Defense Rate. If you don't have Empowered Flame Defense Rate, use Water Elemental.
Frostbite Tower - Empowered Frostbite, Deadly Strikes and Frosty Duet.
Poison Dart Tower - Destruction, Explosive Poison, Defense Rate. If you don't have Explosive Poison or Defense Rate, use Enhanced Poison.
Sky Guard - Destruction, Deadly Strikes, Defense Rate. If you don't have Defense Rate, use Water Elemental.
Spike Blockade - Fortification, Shielding Guard, Sharpened Spikes. Shielding Guard can be swapped for Defense Rate for faster stuns.
Snaking Sands - Defensive Critical Damage, Deadly Strikes, Defensive Critical Strike. Although not needed, a Defense Rate shard can be used in place of Defensive Critical Damage.
Flamethrower - Medallion, preferably with health and speed but crit and speed is good too.
Frostbite Tower - Totem of any variant.
Poison Dart Tower - Medallion with either speed & health or speed & crit.
Sky Guard - Medallion with either speed & health or speed & crit.
Spike Blockade - Totem with speed & def power or speed & crit.
Snaking Sands - Mark with speed or Orb.
Where you'll want to be to deal with assassins.
A decent spot but carries the risk of assassins hitting your towers.
Not recommended for dealing with assassins as it poses the greatest risk of destroying nearby towers.
A good spot if you're good at keeping assassins off you and can stunlock them.