Even with a guide, this is a difficult one.  Just warning you this may take a few attempts.  The last few rounds are particularly punishing.  It will help if you have at least one Baby Ninja and at least one Ninja Dropping.

Wave 01
You begin with a Wee Ninja hut on square 1 and a Forest Ninja hut on square 2.  You should sell the other Forest Ninja near the end of the path.  Build Anti-Ninjas on squares 3 and 4, and a Wee Ninja on square 5.  Fast-forward to start the wave.  At the end of the wave upgrade your Anti-Ninjas on squares 3 and 4.

Wave 02
Build an Anti-Ninja on square 6.  At the end of the wave upgrade your Anti-Ninja on square 6.

Wave 03
At the end of the wave upgrade your Anti-Ninja on square 4.

Wave 04
At the end of the wave upgrade your Anti-Ninja on square 3.

Wave 05
At the end of the wave upgrade your Anti-Ninja on square 4.

Wave 06
At the end of the wave upgrade your Anti-Ninja on square 3.

Wave 07
Build a sniper on square 7.  At the end of the wave upgrade your ninjas on squares 2 and 7.

Wave 08
Through the round you should upgrade your ninjas on square 2 twice, and upgrade the ninjas on square 7 twice.

Wave 09
Upgrade your Anti-Ninja on square 3 when it is safe to do so.

Wave 10
At the end of the wave upgrade your Anti-Ninja on square 4.

Wave 11
At the end of the wave upgrade your ninja on square 6 twice.

Wave 12
At the end of the wave upgrade your ninja on square 6.

Wave 13
Upgrade your ninja on square 2, and at the end of the wave upgrade your ninja on square 7.

Wave 14
Build a sniper on square 8, and at the end of the wave upgrade your sniper on square 8 three times.

Wave 15
At the end of the wave upgrade your sniper on square 8.

Wave 16
At the end of the wave sell your Wee Ninjas on squares 1 and 5.

PAUSE THE GAME, RIGHT NOW!  Read the rest of this guide to prepare yourself - there is A LOT to do in these last few rounds, and time is short.

Wave 17
While your Forest Ninjas and Sniper Ninjas deal with the flying demons, build Anti-Ninjas on squares 1 and 5.  Upgrade your ninjas on square 1 four times, and upgrade your ninjas on square 5 three times.  At the end of the wave sell your Forest Ninjas and your Sniper Ninjas from squares 2, 7, and 8.  Upgrade your ninja on square 5 as soon as funds allow.

Waves 18 & 19
At the start of wave 18 build an Anti-Ninja on square 2.  Through the rest of the wave and through wave 19 upgrade your Anti-Ninja on square 2 to its maximum, and build and upgrade three Anti-Ninja huts in any of the remaining squares at the top of the map.  Two of those three huts should end up five-stars, the other will end up 4-stars.  Placing a Baby Ninja near the start of the track when wave 19 begins should give you enough time to build and upgrade these huts if you are quick.

Wave 20
Place a Ninja Dropping near the start of the map as soon as the wave starts.  Use your Wake Up Call special power to give your Anti-Ninjas a much-needed speed boost.  If the Boss Tree starts breeding demons, use your wind to blow the demons as far back up the track as you can so your Anti-Ninjas aren't distracted by them and can concentrate on killing the boss.  If you need to, and you have one spare, place another Ninja Dropping to help weaken the Boss.