(spoiler)
it's not mentioned but you can deduce it :
- "I had Hera draining the enemy monsters’ health using Absorb, but that wasn’t effective against the Pharolith the way I expected it to be." = absorb, a
jupiter move, isn't super effective on a on a seemingly saturn type monster, meaning there's another type that counter the jupiter moves (so either Mars or Mercury) - "Harper told me I shouldn’t be afraid to use Mars moves against [Pharolith]" = mars move should be ineffective to a saturn type, so if Harper is telling this, then the dual type should be vulnerable to mars moves (so either Jupiter or Mercury)
By correlating the two hints, you can guess that Pharolith is a dual type saturn and Mercury