There are many fanfics that make Ranma extremely powerful. The vast majority of them are not, what I would consider to be, very good. There is nothing wrong with writing about an extremely powerful character. The problem is most people seem to not know how to do it, or at least not how to do it and keep the story interesting. A character that defeats everyone easily is boring; stories need strife and drama.
The eponymous character in the Ranma ½ manga is in many ways god-like/over powered. He is a teenager who hasn't reached physical or mental maturity with the skill to go against people with decades more experience than he has, he learns techniques very fast (just seeing the Umisenken once was enough for him to copy it), he can alter techniques on the fly to suit his current needs (such as the mokotakabisha and Hiryu Shoten Ha), he is good at most of the things he tries to do, he has high physical stats (is extremely fast, strong, tough, has massive stamina, etc.)...
There are many ways to limit a god-like character or otherwise make stories involving one more interesting. One way that isn't very imaginative is to make his opponents god-like as well. Another method is multiple opponents. Have a character fight enough opponents and eventually he will make a mistake or be over powered by sheer numbers.
A good way to limit a god-like character is to give them morals. Better when the opponent does not have those same morals because this forces an over powered character to hold back from fear of hurting opponent and puts the character on a more defensive battle. Ranma has morals that have limited him a number of times. If Ranma did not have morals a lot of his battles would have gone much differently. One example is the breaking point battle, it would have been much easier for Ranma had he been without morals. Ranma was fast enough and skilled enough that Ryoga couldn't even touch him. Ranma put himself in danger because he was unwilling to really hurt Ryoga. Ranma could have easily attacked Ryoga in more vulnerable potentially lethal places such as super speed kick to the balls or finger jab to his eyes and Ryoga would not have been able to do anything to stop him since Ranma was fast enough that Ryoga couldn't even hit Ranma (exaggeration but still fast enough Ryoga would have massive difficulty countering).
Another method to limit the character is honor. Have the character have some type of honor code that causes them to fight at the opponents level or limit themselves to the opponents style. Ranma for instance generally fights at around his opponents level (he could have simply over powered some of his opponents like Tatewaki). Ranma will also allow himself to be limited to fighting in his opponent's style like how he limited himself to martial arts gymnastics in his battle with Kodachi.
Over confidence is another way to limit a character. If the character is over confident he will not take his opponent as seriously as he should and may hold back when he shouldn't or play with his opponent. Ranma does these things. He plays with Akane and Tatewaki Kuno instead of hitting them (Tatewaki Kuno he spent the time to write an insult on his forehead in bruises instead of knocking him out). Ranma held back against his opponents frequently even Saffron trying to not hurt him at first. Ranma has underestimated some of his opponent's abilities.
Distraction is a good way to deal with over powered characters. Keep them off guard or otherwise distracted so that they don't pay enough attention to the fight. There are many ways to do this. Keep them distracted because they are worried about someone. Distract them by using a cheap shot like yelling that something is gone then hit them from behind (or have someone attack them from behind while they are focused on the person in front of them). Have them busy doing something else at the time. Ranma is distracted fairly often in the manga. In the martial arts cheerleading match he was too busy denying his love of Akane to pay attention causing him to get hit in the head. In his first battle with Ryoga he spent more energy and time arguing with Akane than he did on the fight with Ryoga. In one of his battles with Tatewaki he was distracted by a picture.
Being really powerful, having superior numbers, being strong, whatever is good but good tactics could still cause you to lose. Using the environment and setting up traps is a good strategy that Taro used against Ranma. Using sleeping powder/gas on an opponent is good tactic that could take out a more powerful foe. Exploiting character flaws and weakness in some way like how Ranma flashed Herb to get an advantage. Making use of ambushes, tactics considered to be underhanded, etc. Use multiple tactics keeping the opponent off guard.
Weakness. Give them a weakness or fear. Superman has Kryptonite, Ranma has fear of cats, Batman tries to keep his secret identity secret, etc.
What's the point of the battle? Have the point of the battle not being to defeat the opponent. People fight for a lot of reasons, knocking out (killing, disabling, etc.) the opponent is not always the goal. Instead have the character have a different goal such as figure out a technique (or fighting style); defeat the technique, not the opponent; restrain the opponent; humiliate the opponent; obtain an item from the opponent with out injuring them; etc. Example: Ranma's goal wasn't to beat Akane unconscious when she was wearing the dogi it was to hit one spot and no other.
Have the person not wanting to fight the battle in the first place this would generally limit people. For example, Ranma did not want to fight Konatsu but was forced to.
Another way, not used as frequently as I would like, is to show the negatives. Super strong characters have to be careful or they would break stuff/ people by accident. Fast characters would bore easily and be impatient (think about it waiting in line upsets regular people to a person who regularly moves at super speeds it would be torture). And so on...
Have bad stuff happen cause of winning or because of the god like abilities. Character won. Now the character is the ruler of the area and will have to deal with all kinds of problems like assassination attempts, how to rule the people, listen to peoples requests, marry the neighboring kingdoms daughter, etc. Ranma had this in his defeating Shampoo. Powerful enough that people actually start to worship the person and having to deal with all the problems caused by that.
Wear the over powered character down prior to the battle. There are many ways to do this. Prevent him from sleeping, have him fight a lot of people before getting to the main fight, drug him, starve the character, etc. Several of these were done in the Ranma series. Several characters used drugs, Ranma spent the entire night prior to the fight with Kodachi fighting with Ryoga, Ranma starved himself during the martial arts eating contest, etc.
Have the god-like abilities be entirely useless in the situation or a hindrance. God like ability to fight - Not that great for peace talks. Multiple god like abilities - that help you not at all when dealing with an upset girl. And so on... Many god-like abilities - point of view so different from normal humans that they have an inability to understand normal peoples point of view and can't relate with normal people. Minor example: A character that runs into ghosts and spirits frequently probably wouldn't understand why people would fear them.
There are some things that you really should not do when creating or writing about an over powered character. Do not remove all the characters faults; if you really want to then create new ones. If you read about mythical gods the majority of them have quite a few character faults. Do not have the character win every battle easily as that is boring. Do not have the character fight progressively more powerful characters. Well that is alright for a video game; it doesn't really work that well in stories. It is usually significantly better to vary the opponent's strengths, use tactics, and limit the main character, etc., than go the progressively more powerful route.