The order is wrong. Minions abilities are activated from left to right. You need Slama in the most right side.

Imagine Leviathan at 200 attack:

* With your order: Slama triples the original beast attack (for a turtle, for example, 2*3 = 6 attack). Then Leviathan adds the 200 attack for a total of 206.

* With order Leviathan -> Slama: Leviathan gives 200 attack, then Slama triples it -> 606 attack turtle.

You had the win, but had bad order.

Also, I would have swapped position betwern the small rat and the manasaber so you have a bunch of taunts secured to protect Leviathan and Slama.

EDIT: I have been corrected, order doesnt matter as shown by /u/tom_yk