finally............beat the boss at level 10..... here is how u can do it .
1 Upgrade your sprout,lightningand mana to the max
2 Use fire archer towers at the entrance
3 Use the sprouts to create a buffer and push the boss back
4 Use the lightning to hit the boss ( Sometimes the lightning WON'T electrocute the boss )
5 Keep repeating the same to sap his health.
6 Build lots of archer towers around the castle, near the boss and upgrade them to the shooters which have a long range ( these will kill ALL of the spawns of the boss )
6 Patience is a key thing here.... i beat the boss only after 30 minutes ........
7 Follow these tips and you will beat the level with 3 gems like i did!!