Here is a guide for people who struggle about how to defeat challenge at hard level to unlock endless level without using clovers. check the screenshot at end to understand placement of towers better.
1- place a slinger in between spawner and destroyer area so that it touches three possible paths
2- upgrade slinger to 2
3- buy a crossbow and place it below slinger and breed it to max level
4- once its max level move crossbow to middle
5- repeat breeding crossbows for 3 times and place them on middle
6- you don’t have to buy anything from now on till defeating slime king.
7- after defeating slime king, unlock water
8- Buy 3 water towers max them
9- Buy 1 ice tower and max it
10- Buy 1 fountain and place it in middle leave it 1 level for now
11- Buy 3 more ice towers
12- Meanwhile you will defeat centurion very easily.
13- Maximize fountain
14- Unlock stone and start to breed 4 max acid towers
15- Then fill the area that is covered by fountain with 3 level 2 stone towers
16- When you defeat summoner, unlock energy and get emp… place it near fountain
17- Buy 1 more 3 level stone tower (you should have 4 at this point)
18- Level emp to 2
19- Buy 4 stone 3 level (8 at this point)
20- Max emp to 3
21- Buy volcano and max it
22- Fill the rest of the squared covered by fountain with stone 3.
I knew it was going to be host+nanobots it was just matter of time... that's the most difficult wave i found...
