When I personally tested and passed (also quite easily at Nightmare difficulty), I used:
8 Slots unlocked:
Illustrated Book, Slime Catapult, Miner, Double Dino, Smart Alien, Kamikaze Bear, Baby Boom, Toy Car (or DollHouse, as you prefer based on your strategy)
Column 1, 6 Illustrated Books
Column 2, 6 Slime Catapults (upgrade to buy)
Column 3, 6 Miners
Column 4, 6 Double Dino (against Skeletons)
Column 8, all Kamikaze bears you can.
If you are able to well balance your fantasy you could be able to place even 2 Smart Alien (positioned in lines 2 and 5 they shoot on all 6 lines!).
Then BabyBoom and Toy car (or DOllhouse) at need.

Each one of our developer team has his own personal different strategy.
What I can grant is that I passed level 13 and 14 as above suggested ;)
Try and have fun!
Davide
Xplored CEO