If you have a mushroom on your field, try to click
on every skull first. After using the mushroom,
every skull turns either into a mushroom, an
eater, a rotator or a bomb. If you have many skull
on your field, this makes it very easy to clear
it.
But beware not to turn all your mushrooms
into skulls as well!
This is too easy and no developement during the game, which makes it boring very soon.