If any of you guys have read Slime Isekai, you may be familiar with the concept that:
Ultimate Skill > Unique Skill > Extra Skill > Normal Skill
this kind of rule is absolute, which can explain why the crafter which has the (combat oriented: Unique Skill) can deal alot of DMG to the boss. The MC's unique skill isn't combat oriented, hence for damaging opponents he essentially has to rely on combat oriented "Normal Skills" which are inherently inferior to combat oriented Unique Skills.