They either have to take a ton of damage or discard a card for every spell they cast
Druids and merfolk are a very restrictive pair of creature types, but it makes sense when you have such a powerful recursive ability in retrace
She also has a mana ability that costs the same as her casting cost (3G, 1C), which creates X 1/1 Squirrel creature tokens where X is the number of Squirrels you control
For 4 mana, you get the effect of Entomb and 2 copies of Natures Spiral allowing you to set up for some pretty powerful plays
Its the kind of thing that might not seem like much on a single turn but across a game is likely to be the difference between victory and defeat
If you keep casting instants and sorceries, he can easily reach 10/10, and if his power exceeds 21, you can win in one hit