Immediately summon all units you can. If you've chosen the archer instead of the imp, put your priority upon the other units (melee). Otherwhise, put the priority upon your best ones (highest tier). Summon them all on the same lane without letting them change lanes. Don't select any unit (press the unselect button if one is selected)
When an imp approaches, just press the forward button.
When it will be dead, press the backward button.
Continue summoning units. Never stop doing it!
You won't need to use specials. If you have to use the tower elfy's one. When you did it press the unselect button.
If it doesn't work or if you have questions, write in the comment section below.