Learn English – A word/phrase for something massively complex

In some professional organizations, the word "Journeyman" is used for someone who has passed their apprenticeship and able to complete work on their own, but are not necessarily masters yet.

EDIT: If you are seeking a skill level reference for learning material. This seems to be a standard:

For Beginner [programmers, developers, carpenters, chefs]
For Intermediate [programmers, developers, carpenters, chefs]
For Advanced [programmers, developers, carpenters, chefs]

You are asking for a word to replace "intermediate", but the more I think about it "intermediate" may actually be the word you need.

A player who doesn't buckle under pressure, could be said to deliver the goods. And I would also describe such a player as being ruthless and consistent.

However, a slang term which fits the Op's request is clutch:
to perform under pressure In the last few seconds of a close game, only a player with clutch can lead the team to victory. (Derived from the clutch mechanism in a manual car, where perfect timing can mean the difference between a launch and a stall)

A reputable baseball coach has this to say: ¹

If a hitter is averaging .333 and gets a hit one time in three clutch situations, he is performing well in the clutch... normal performance under pressure is the goal, not super performance.

All too often an athlete gets the reputation as a "great clutch player" because of one or two performances that got a lot of media attention, but the truth of the matter is most of the time a "great clutch performer" is one who performs normally under pressure.