Against karthus, Karthus will be looking to farm as much as possible so he can get his Ultimate e as quickly as he can. You can delay his level 6 power spike by invading his jungle and stealing away his camps. 

Once Karthus has his Ultimate e, he can impact the map at any given time. When fighting, always consider that he can use his Ultimate e to turn the exchange around. 

Taking away Karthus’ blue buff will make it harder for him to clear the jungle as he needs as much mana as he can get to clear it quickly. If you know his buffs about to spawn and he’s on the other side of the map, try to steal it away. Against karthus, Karthus will seek to cultivate as much as possible so that he can get his Ultimate e as quickly as possible. You can delay his 6-pitch power level by invading his jungle and flying his camps. Once Karthus has his Ultimate e, he can impact the map at any given time. During the fighting, always consider that he can use his Ultimate e to turn the exchange around.