First, Skullius gathered everything he knew about Ferex's Flaw. The Apostle was not supposed to engage in tough battles. Or more aptly, Skullius had interpreted this Flaw as meaning that Ferex had to have the efficiency in combat to dispatch enemies without effort.
When the Apostle was still the Limitless Body Null Demon Hound, he had quite the inflated stat for Intelligence and truly, if it wasn't for the fact that Skullius' journeys never went in a linear pattern of difficulty according to his growth, Ferex would have been more than adequate with just this.
At least, he was smarter than what Red Rage had been in the beginning.
When Skullius thought a lot about Ferex, he even considered the fact that he had wanted the Apostle to be a speed-type combatant by his side with Red Rage being the brawn. After all, he himself had a wide array of diverse abilities that could complement the two's speed and strength in battles.