Against karthus, If you see Karthus using his Ultimate e at the start of the fight, try to CC him so it cancels his Ultimate e.

If you kill Karthus at the start of the fight, try to reposition and get away from his Passive. This will reduce his damage output and make surviving the team fight easier.

If you spot Karthus in a side lane split pushing, you could start a team fight. But, you need to keep in mind that he can influence the existing fight with his Ultimate e.  Against karthus, If you see Karthus using his Ultimate e at the beginning of the fight, try the CC to cancel his Ultimate e. If you kill Karthus at the beginning of the fight, try to reposition and move away from his Passive. This will reduce his damage exit and make the survival of the team fight easier. If you spot Karthus in a side track pushed by division, you can start a team fight. But, you must keep in mind that he can influence the existing fight with his Ultimate e.