Is AR defense possible against a good player?

Depends, what rank are you? People start getting really fricken cheeky tier 17 and higher. While a flawless victory is possible, if you don't have the units required and/or your structures are in the wrong place you may leave with a unit or multiple units dead.

A careful player with good understanding of AI movement order will most certainly win. The point of making a decent AR defense is to minimize the lift loss by taking down as many of their units down as much as possible. Sometimes, just taking out one key unit is enough for the opposing team's composition to fall apart. But sometimes, a loss is inevitable. The best set up you can do is to make your team in the hands of the AI to be as unpredictable as possible by using knowledge of AI move order and utilizing skills that allow unusual movements. I've seen units with unusual skills like Pass and of course teleporting skills like WoM, Escape Route, Flier Formation, and Guidance adds on to the unpredictability.

Well, if I can say so of myself, I'm pretty much an idiot when it comes to AR.
I possibly lose 1 out of 3 matches that I play daily and mostly lose 1-2 units every fight.
But guess what, it's complettly irrelevant (for me atleast).
Trying to be competitive in FEH is like pulling out a nail out of your foot:

It hurts when the nail is in your foot, it hurts when you try to pull the nail out and it still hurts when the nail is finally out of your foot.
You maybe feel better after removing it, but you'll still have a scar of it to remember you how much it pained you.

The last thing didn't really had substance to the question, but well.

Just have fun Mr.Seeker and don't give yourself a headache for that stupid gamemode (ಽ ͡☉ ͜ʖ ͡☉)ಽ