That just isn't true. You shoot in response to the first spell being played, its already a 2/2. They respond to the effect again its a 3/3. They only need 2 spells.
Not really sure what you are referring to.
Lavamancer seems much better than Sulfur Elemental, itīs cheaper, it effects not only white creatures and itīs a hell for GAT to play a dryad, when your lavamancer is untapped, cause they must have three other spells to let her survive, as you shoot her in reaction to the first trigger.
First triggered ability goes on the stack from them playing a spell, then you shoot the dryad. It's still a 1/1 when your lavamancer resolves unless your opponent play 2 more spells.