The phrase "an effortless one" is correct and usable in written English.
It can be used to describe something that is easy to do or accomplish without much effort.
Example: "Completing the project was an effortless one, thanks to the clear guidelines provided."
Alternatives: "a simple task" or "an easy one".
A likable, shy young man happiest before the altar of his Playstation when not beating the bejesus out of tennis balls, Nadal plays an unprecedentedly violent game, heavy on topspin and sweat, the perfect foil to Federer's effortless one.
"Imagine the Africa of the great safari era, when blazing sunsets melted into lantern-lit romance and service was an effortless whisper," declares one, and it is typical.
Blue eyes and tan skin in an effortless swimming motion, one with the water.
In the first half of the three-hour program, Mr. Maharaj performed a series of expressive evocations of Krishna's childhood, one involving an effortless transformation from a naughty little boy stealing butter to the milkmaid who chides him.
He gets another three more fours too, one with an effortless flick off his legs down to long leg, and another with a thumping pull past wide midwicket, and is similarly punitive towards one pitched further up, driving it confidently through the covers to the boundary once more.
For some, the culinary dance might be an effortless and ingrained tango -- one where each step is steady, instinctive, and flawless.
Moreover, resubmitting a new job from previous graphic results to remove rogue taxa and add new ones is an effortless task when using PalmTree.
It's even there in the gleeful cruelty of Scaramanga's dwarf sidekick, Nick Nack (an effortless assemblage of offensive notions in one sketchy character), who hires killers to take out his boss, with the boss's full approval.
And while being a woman may never be an effortless undertaking, it could and should be one of equality.
Everyone knew her from school - she directed the plays in grade school, would take in kids who were going through divorces, knew and loved and was not shy about hugging any of them, especially the shy ones - there was an effortless kind of understanding, an utter lack of doubt about what she was doing that put people at ease, so unlike some of the mothers, so brittle and unsure.
