This is dealt with in the episode screenplay. The Rocha creature attacked Lt. Aquiel. After it had begun absorbing her memories but before being able to replace her, she was able to fight it off and grab a phaser. After unsuccessfully using it at the default setting, it was then reset to level 10 where it was used on the shape-shifter. The phaser was then fired for a sustained amount of time before she was satisfied that she'd killed the creature. Unbeknownst to her, the creature was not dead and was able to shift into
RIKER: The phaser was set to level ten. (beat) Standard procedure requires that all phasers are set to level one when they're
TNG: Aquiel - Original Screenplay
RIKER: Doctor Crusher tells me that in order to inflict the kind of molecular damage found in Rocha's remains... it would take a
sustained phaser discharge of at least thirty to forty seconds.
When he's attacked Geordi goes through the same process, initially firing his phaser on stun, but then, using his knowledge of the creatures strength, he moves his phaser up to nearly its highest setting, level 14.
Although this is a more risky move (the shot could potentially blast straight through the ship's hull or hit a vital piece of machinery), he knows that the creature has a natural immunity to phaser effects.
The creature makes a final LUNGE. Geordi jumps toward a nearby desk --
opens a drawer and grabs out a phaser. He aims at the creature and
The creature is JOLTED back -- but quickly recovers and moves to
attack again, roaring, furious.
61A INSERT - PHASER
as Geordi rapidly taps the phaser, adjusting the force setting to
The phaser rapidly kills the beast without needing the same sustained blast.