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.
I play Karthus always in the middle.
I constantly watch the enemy to find out how he behaves in.
And I always try my small bombs hit the enemy.
If the opponents I play aggressive defensively and stay near the Towers.
Once you reach level 3 you can surprise the enemy.
You wait until he is close enough and then set purely your wall. Now it's on it with Defile and always put your bombs.
Modest given the players you have reached level 6.
You need to look at the map to see when the Ultimate to be set. In addition, always nice to go gank the site Lanes. So you gather with your Mejai's stacks.