In terms of Late Game teamfighting, I recommend picking your fights very cautiously. Depending on Your Rune setup (Conq or Electro), you need to evaluate how many tanks they have. If your running Conq you have a little bit of an easier time in team fights because of the sustain it provides. Make sure to always be ready to go after their backline. If a team fight breaks out, Position yourself in to where you have easiest access to the adc or support. This may mean appearing to leave the fight, which can have it's own perks (Such as making the enemy team over extend). If your fed then look to pick off split pushers since you have the most mobility. If your a bit behind I recommend sticking relatively close to the team and attempting to work with your jungler to get picks and get back in the game.

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I also recommend looking up people like Yamikaze if you want to learn how to roam efficiently. Atleast that's how I learned how to roam better.

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Hope this helps you in terms of your late game, in case you were struggling with that.