The arachnid wing druid class challenge is very fun because it forces you to focus on buffing your monsters.
Strategy 1: Stone Gargoyle
If you get a stone gargoyle early on, you are bound to win. This seemingly weak monster will soon be your path to invincibility. If you get a Wild Growth and/or a Mark of the Wild, use them to pump up the gargoyle's HP. As long as it can withstand a round of attacks (~5 damage), it will never die because it will heal at the start of your turn. With Mark of the Wild, it will then become a wall that your other creatures can sit behind.
If you get a Nerubian Egg and a Defender of Argus, use the defender to put taunt on the egg. The egg will then become the focus of the computer players' attacks and you will soon have a 4/4 creature to help speed up the enemy's demise.