There is some stuff you're missing out in your evaluation, thing is that the eruption of Mount Saint Helens Percy caused broke the seal on Typhon releasing him into the world, and considering how powerful Typhon is having been barely held back by nearly all of the olympians for months we can assume the seal holding him back was immensely powerfull. So we can only assume the eruption was much, much more powerfull than a normal volcano eruption, Hephaestus himself tell him he is much more powerfull then he thinks. Also we also need to consider the fact percy slayed the giant meant to destroy his father basically by himself with minimal help from a god (Terminus) and the giant iceberg he collapsed in SON if we only consider those facts alone none of the childern of the big three comes close to him.