The trick with realism in fantasy lies in the rules you set up for that setting. This is why world building is incredibly important and why someone saying “I can do what I want because it’s just fantasy!” is both true and ultimately false. A character who exists in a world that’s very similar to our own will function under similar rules to our own world, a character who exists in a fantasy setting will fall under the rules that have been set up for that world. Fictional worlds require rules because they create tension. In a lot of ways, regardless of whether your character is handling a firearm or a fireball, what makes or breaks a story are those rules, how well you as the author set them up, and how well you adhere to them.
What a character can and can’t get away with will be defined by those rules. Now, superpowers whether they are magical or natural add an extra challenge because they ultimately raise the stakes. Magic will get away from you very quickly if it’s not balanced by cost in a fictional world. A character who can do anything, but has no restrictions, ultimately ends up boring because we as the audience has no reason to care about them or worry about whether or not they’ll succeed, whether they’ll survive the next ten minutes.
Realism begins in the restrictions you place on your setting, in the cultures you craft, and how your character reacts as a product of that culture. It’s in how well your plot and character actions sync up in the world you’ve created. After that, the realism of the fight scenes is like icing on top of a very well-made cake. If you don’t have that, then it’s just icing and while great icing is delicious on it’s own, it pales when it compliments a great cake.
I’m actually really hungry right now, I don’t know if you can tell.
Anyway, you have to make sure you don’t let your imagination run away with you and because it’s fantasy, it is ridiculously easy to have that happen. Now, the easiest way I’ve managed to start setting rules (and it’s hard) is to take an RPG system I’m familiar with like GURPS, World of Darkness, or Exalted, pick powers, and just start plugging in numbers. The White Wolf ones are good over D&D because they spear head character development in conjunction with the superpowers. Now, you know what your character can and can’t get away with. Stick to that and follow those rules, track their trajectory and always craft antagonists who are their equal or better. The antagonists will require their own character sheets, even the throw away mooks your character is fighting.
Why? Because every character in the story is there for a reason and a fight is between two or more people. So long as the fight stays within the setting established rules and the character stays within the themes the story has set up, then it’ll be fine. It’ll feel real to the reader. Upgrade your understanding as needed. Be careful. And you’ll be fine.