I don't think Xander had done anything necessitating redemption, although there are people who think of normal adolescence as a sin of some sort.
The first time we see him act badly is in "The Pack" but this is a spell that only hit him because he was coming to the aid of another student who was being bullied.
In nightmares, we see Xander being the only one to destroy his fears when he punches out the clown. He never has that fear again even though all the other characters are still plagued by their fears.
By the end of Season 1, he resuscitates Buffy and without him, it would have been a very short series.
In Bewitched, Bothered and Bewildered, he tries to actualize everyone's adolescent fantasy - he tries to cast a spell that makes him irresistible to the opposite sex. Being able to accomplish something that everyone else would want to do but can't is not a sign of something needing redemption, just a sign that he is still learning. He is given the opportunity to act on his sexual fantasies about Buffy but he declines.
When Buffy has to deliver the knockout to Acathla, Xander is the one to get the rocket launcher.
Dead Man's Party is where he comes off at his worst and most judgemental. If there is an arc, this is a notch where it goes down.
Next, we get to The Zeppo. "I like the quiet." And all the turmoil and Brownian motion in Xander's mind goes quiet and stays that way. Plus, he finally ends his loser status with is refusal to answer Cordelia's taunts.
Later, we see Xander, long-time resident of loserville, being hooked up with Cordelia, prospective prom queen. It goes off the rails, but not every hookup is intended to last and he ends up buying her the prom dress she couldn't afford on her own.
Xander then becomes General Xander under Field Marshall Buffy when the Mayor goes boom.
In "Something Blue", Willow casts a spell to make him a demon magnet and after that, all of his lovers are demons. The spell never gets fully reversed. I approve of him ending up with Dawn in the comics - their relationship is based on mutual respect.
If there is an arc, it does have a step function at "The Replacement" where Xander characteristically tries to deflect the energy cast at Buffy and separates into the competent Xander and the incompetent Xander. By the end of the episode,
In "Flooded", he gets Buffy the best deal he can on replacement plumbing.
In "Hell's Bells" he hits a low spot again, but this is because a former victim of Anya's comes back to take revenge. Xander's only flaw is that he didn't dump her sooner.
In "Grave", he does an outstanding job of saving the world using love rather than powers or weapons and this is one of his peaks.
In "Selfless", he tries to get Buffy to reconsider killing Anya.
In "First Date", he is still a demon magnet.
He is the one who recognizes that Spike is being triggered.
He fights Caleb and loses an eye.
I don't think Xander has a smooth arc, but I also think anyone who sees him as needing redemption is someone who hates adolescents in general.
