Renekton is a burst-based champion that uses a differant type of energy source called Fury. This is NOT similar to Tryndamere's Fury. When Renekton has 50+ Fury, his abilities, aside from his ultimate, give him bonus benefits. His ultimate will give him a chunk of health AND generate Fury per second. It will also damage nearby enemies for a percentage of ability power per second. Although this is true, building AP Renekton is never worth it. Heres how I play solotop Renekton. When the enemy champions have to move back from the tower area, due to their lack of minions, I activate my Ruthless Predator, and lead in with Slice and Dice, to the nearest opponent. I MAKE SURE I hit Slice, as to be able to get away with Dice. After I hit the enemy with Slice, I basic attack them for the high damage on Ruthless Predator. I then Dice away. This is for harassment purposes. If theres an enemy with low health, and they're lingering in lane, but I can't quite seem to get to them due to the other enemy champion, I Slice through the protecting champion, and Dice to the low-health champion.
Keep in mind that Ruthless Predator does the most damage, out of all of your abilities, with 50 Fury and without 50 Fury. It also stuns the hit enemy.

If enemies have high CC in my lane, especially when their disables have range, I tend to play defensive. Defense is a form of offense in League of Legends. It allows for easy ganks from your jungler. It also makes your enemies annoyed that they won't be able to kill you or easily get your tower. Always stay a fair distance away from your enemies if they are on their side of the lane. Make sure you keep your eyes and reflexes aware for stray CC abilities or for champions going out of the wave and coming at you. If its a skill-shot CC, dodge it. If someone has a targetable CC, its more than likely short-ranged, so when you see the enemy champion moving towards you, move away from them. Always make sure you're in enough range to get EXP from your minion's kills as well. Now, what do you do if they reached your tower? If theres a ranged attacking it, drive them away first. If theres a melee attacking it, save them for AFTER, unless the ranged is an AP or a support. Thats when you attack the melee first. Now, keep your eyes on the map. Look at where your Jungler is. The best thing to do for ganks is to ask your jungler, at the beginning of the game, to ping whenever he is ganking. If you can get the kill, GET IT. Who cares if your jungler did all the damage? You're going to NEED THAT KILL. EAT IT. EAT AND SHOW THE ENEMY CHAMPION THEY WERE OUT OF THEIR MIND WHEN THEY THOUGHT THEY COULD HANDLE A SOLOTOP RENEKTON. EAT THEIR SOUL AND DEVOUR IT, THEN POOP IT OUT SO YOU CAN EAT IT SOME MORE.

Just a quick note; Renekton's Fury is not the same as Tryndamere's.

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Cull The Meek: Gives you extra damage and extra health regain when you use it on enemies. Use this on large crowds of minions, as it will regenerate alot of your health and pretty much kill the entire wave if used right.

Ruthless Predator: Increases the AD Percentage for damage from 150% to 250%, and increases the stun time to 1.5 Seconds. This is your heavy hitting, champion-killing move.

Slice and Dice: This move only benefits Dice. It increases the armor reduction on hit and increases the damage.

When these benefits are used, they consume 50 Fury. It is not optional to consume 50 Fury or not if you have the Fury.

Nasus is the type of champion, when soloing top, who is forced to focus on the closest enemy. Your goal is to farm your Q alot. And I mean ALOT. Finish off minions with your Q if you can. The reason this is is because it gives him bonus damage on his Q. If you're going to harass enemy champions, slow them then hit them ONCE with your Siphoning Strike. Use your Spirit Fire to bring down the HP of a large group of minions. And, for whatever reason, if you happen to push the enemy to THEIR tower, use your Spirit Fire RIGHT INFRONT of their tower. This way, they can either choose to take damage from your Spirit Fire, or avoid the damage and let the tower kill your wave of minions, losing them gold. So, its pretty basic. Last hit with Siphoning Strike, Wither enemy champions to use Siphoning Strike on them, use Spirit Fire to deny gold. Nasus' Ultimate is similar to Renekton. He gains a percentage of health, and deals magic damage based on ability power to nearby enemies. However, the damage is a percentage of an enemy's max HP. This makes this ability much more useful than Renekton's ultimate. Although this is, some people prefer Renekton, some people prefer Nasus. I like both their playstyles.

This is pretty simple. Use Spirit Fire to clear large waves of enemy minions, Wither enemy champions if they dive you or harass you under your tower, and farm your Q whenever possible. When your jungler comes to gank, Wither the target champion.

Teleport allows you to get back to lane quickly.
Ghost allows you to get back to your lane quicker, but not as quick as Teleport.
Heal allows you to 2v1. Sometimes.
Ignite allows you to finish off enemies if you really need the kills.
Exhaust can help you get away from enemies or catch up, but not get back to lane faster.
Flash can help you get away from enemies or catch up, but not get back to lane faster.

Now, if you hadn't a certain champion in mind already, you may be asking yourself; "But which one should I play?"
Well, this brotherly duo have different type of playstyles. Nasus is more of a defensive gesture, while Renekton is an aggressive champion. Nasus players tend to look at how fast or slow an enemy can get away, not the damage they're dealing. Renekton players tend to not care about the enemy's HP is, but how much damage their moves can deal to that HP. Although they have similar abilities, they both share mainly one thing: No, not relation, (but yes, they are related. I know, what kind of messed up parents have an iguana and a chihuahua as children?) They both share power. They both can be aggressive, and they both can be defensive. This gives them POWER over other champions. This gives them a WIDE range and expanse of builds. Choose which one you think you'll like better.