Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is my set in stone Karthus build. I will put more into the description as right now I don't have the time.
The build is fool-proof though. As long as you don't feed in mid-lane to anyone, you should have the range to keep on the back lines while poking them.
Keep in mind you can cast behind the character where they are standing so they don't see the bomb, and can not dodge it in time.