Easy is to beginner as medium is to
I think the correct word is intermediate, as @Rathony mentioned. It's also a noun, so He is an intermediate is perfectly OK:
intermediate (noun): one that is in a middle position or state.
(AHD)
intermediate noun [countable] [plural intermediates]
a student, player etc who has not yet reached an advanced level
(Macmillan Dictionary)
One classic example is Minesweeper:
@Rathony If you choose to post "intermediate" as an answer I will happily delete mine or edit it to suggest other alternatives.
UPDATE: Some EL&U users have challenged the use of intermediate as a noun. Some went as far as to say that "I'm an intermediate" have never (sic!) been said.
If you doubt this usage, consider this entry from the Oxford American Dictionary:
Intermediate is used awkwardly as a noun.
It's most often an adjective as in:
"...intermediate English..."
And when it's used as a noun it most instinctually means a negotiator. Someone who serves as an intermediate between two parties in conflict.
I would recommend the use of journeyman.
It doesn't fit perfectly because of the implication that they will not necessarily improve, but in the history of the word in use, many journeymen have excelled beyond their perceived abilities and become champions, or experts.
Moderate may fit your needs in terms of difficulty.
average in amount, intensity, quality, or degree.
You could also change the whole system and use amateur, veteran, and professional. This system is all based on nouns, and can be generally compared with experience and pay.
Amateur
engaging or engaged in without payment; nonprofessional.
Veteran
a person who has had long experience in a particular field.
Professional
a person engaged in a specified activity, especially a sport or branch of the performing arts, as a main paid occupation rather than as a pastime.
Adept (Virtual Villagers Skill Level)
- Begineer
- Adept
- Mater