I could not get to "Ravana" (the boss) by turn 7, but did it on turn 8. I followed Dionysiosk74 method, but never killed him/her, even with 5 or 6 tires.
So, used his plan (thankyou) but gifted "Dambulla" instead. The 2 airlifted units were left in defense.
Turn 3, deploy 1 to Uda-, attack south. Leave 2 at Laggala
Turn 4, Deploy 1 to Laggala and 1 at Mahi..., attack SW
Turn 5, Deploy 2 to Laggala, attack with it N and East, 2 each. Attack West with Bibile
Turn 5, Deploy 2 to Laggala, to hold the bonus. Add 1 to Monaragala, attack NW.
Turn 6, Deploy 2 Laggala, moved some units back there too. Attack NW from Monaragala
Turn 7, Deploy all to Wellawaya, move What you can to Matale. Attack west with Wellawaya.
Turn 8, Had 6 to Deploy to Tissa..., then did the order delay?? and attacked North to boss with 13 units...
Why did I include so much detail? For those having tons of trouble, you may just want to get through the level. Briefly read the plan, if you don't want so much help. And, don't watch the video! LOL