What’s the toughest skill for L6+ engineers that take long time to master?

Great question!

Looking back on my journey as a SWE, I was a quick learner and had decent technical chops by the time was an L4+ at Google. Around that time, I learnt the concepts of extreme ownership and program management skills that lead me to becoming L5.

Now, my path from L5 to L6 was a combination of doing what I was doing but at a larger scope (so bigger team, harder technical problem etc.), but an additional critical skill which is often ignored by TLs: team cohesivity and growth.

• You deliver faster: your team, like a well-oiled machinery is productive, is free of interpersonal conflicts and individuals co-operative and helpful to each other. All critical parts of recipe that delivers faster.
• You scale yourself faster: if you can grow your junior eng into senior, you free up time for yourself to tackle even bigger things.
• You quickly become a valuable TL to managers/senior leaderships -- you want to get their reports achieve their goals (promotion, work-life balance, fulfillment)!

I get into more details of this skillset (with some more anecdotes) in the Team Unity + Growth section of my course for TLs: https://www.jointaro.com/course/tech-lead-blueprint-role-skills-and-strategies/empathetic-leadership/

Fwiw, the whole course is useful for learning behaviors and leadership skills that L6+ engineers need.

There are many skills that are needed for staff+ engineers, and I like what Lalit has already mentioned.

One of the most difficult I have found to master (in my own experience and in the experience of people I have worked with) is handling conflicts and helping people align in one direction.

The reason the skill of conflict resolution and people alignment is hard is not that there is anything fundamentally hard about it. But because during such discussion, our emotional side get activated and our rational side become weak (System 1 dominance in the language of the seminal book Thinking Fast and Slow). When that happens, we are literally unable to "think through". Our reptile brain takes over.

There are many formal and informal practices that I adopt when working through this skill, but those are likely outside the scope of this question. Hit me up to discuss if you are finding it challenging.