If you give a character a new skill or an OC a skill, there are a few things to take care of. Especially if you want to avoid making them a Mary Sue or Gary Stu.

Never have a character save a situation with a skill that wasn't shown yet. It makes them look like a Mary Sue. (I use only Mary Sue, because one name is enough, I do not say this problem is exclusive to female characters.) And it is just lazy writing, you may be accused of just throwing the skill in out of nowhere. Show the skill way before it comes to use, show it again though the story.

Also worthy of mention when you want to avoid them looking like a Mary Sue, show them learning the skill, or at least show them progressively getting better with this skill. once again, it is important how and why they got these skills. Deus Ex Machina Skills, especially strong ones, are just boring to read, lazy writing and bad story telling,

An example would be. Lets say X often notes little details, like small sounds. Show them hearing something dropping onto the floor or a small animal making a sound and later, they are in a laser gun battle and hear the opponent's weapon click, signaling that their ammo is empty. The character knows they won't die and uses this to attack the opponent physically. The opponent is confused why the weapon isn't working and is distracted enough for X to beat them.

In general, set things up, introduce everything before it is important, to avoid Deus Ex Machina/Mary Sure moments.

In the same vain, I want to say, show, don't tell. Don't tell how great of a swordsman Y is, show us how Y kicks ass using said weapon.

In general, don't make the character a being that solves every problem. Without struggles, the stakes get meaningless. With meaningless stakes, the story gets boring. This is the problem with Mary Sues. Their stories are boring because their is no struggle the reader could get invested in.