Saving EsmereldaEsmerelda is one of the easiest bosses, just as long as you know exactly what to do and what you're looking for.
As the level starts, switch to Bungalow and jump up the path and watch the opening. Once Esmerelda begins to leap forward, run behind the beach ball or avoid the shots Esmerelda sends out. Now move around and look at the float she is standing on. There should be a little pump at the front. Wait until Esmerelda needs to recharge, as there is a brief moment where her shooting comes to a halt. Shoot the transparent pump only once, and she moves to another float.
Again, avoid her continuous shots and look at the float. There should be another pump a little further back. Shoot it, and she jumps to a third float. Beat her here and she goes to a fourth and final float. If you need to refill, there is plenty of ammo and Pet Yums scattered around the beach ball. On the final float, she will fall under the lava. Do not hit the lava yourself, or you will automatically fluff it, despite your success.