Against karthus, Once Karthus has his Ultimate e, he can impact the map regardless of his positioning. Whenever you’re fighting the enemy, always take into consideration that he may use his Ultimate e to finish you off.

The more AP and the more magic penetration Karthus gets, the better and stronger he will be. Try to shut him down or constantly invade him to reduce his gold income.

The longer the game goes, the stronger Karthus is going to become. Ensure you put him behind in the early game to make him less useful as the game goes along. Against karthus, Once Karthus has its Ultimate e, it can impact the card regardless of its positioning. Every time you fight the enemy, always take into consideration that it can use its Ultimate e to finish you. The more the AP and the more magical penetration Karthus gets, the better and the more it will be. Try to close it or constantly invade it to reduce its gold revenues. The more the game goes, the stronger the Karthus will be. Make sure to put it behind in the early game to make it less useful as the game goes.