Upgrade speed and damage of gun tower (10) of each will be enough.
Then put 1 tower on first corner, so it can shoot 2 times. When you get enough cahs build another gun tower next to him, so you also hit 2 times.
Then build in first corner shock tower (third tower) to kill all butterflies. After that upgrade shock tower atleast 2 times. Other upgrades you can spent on gun or shock tower.
 "Andaho"){kind=small}
 "Butalc"){kind=small}