Alverto's death may seem sudden and anticlimactic, but it was not so easy or so abrupt.
He was first severely weakened by Jin, who nearly sacrificed his life and future to weaken the link between Alverto and the spire of ascension.
Next, he had to fight Link and the others while being bombarded with immense amounts of purification and healing magic by Riya constantly.
He even had to burn his own soul to forcefully teleport them, which weakened his soul and reduced the power of his 'Soul Resonance.'
Then he had to burn his 'soul' again to deal with Rio, making him even weaker.
He was hoping that Rio would get hit by the 'memory' trap and would not be able to reach floor 0, but that plan didn't work at all.
Alverto was already out of options and was at his wit's end.
He kept getting weaker and weaker with each wave of enemy while Rio got tremendously stronger.
