I show you two possbilitys how to skill shaco, cause you can skill him ap and ad. My personal opinion is that ap shaco deals much more damage because of his e skill and the jack in the box.
In mid game and in the beginning of a game shaco is the most op champ you can play with.
With cooldown reduction you can use youre e skill every 4 seconds and so they can't win 1 on 1 in mid game. You can port you with your q skill behind him and then you attack him, so you deal critical damage and bonus damage because of youre passive skill. When he wants to run away you use youre e skill to slow him and so you can attack him until he dies. Because of cooldown reduction you can use youre q again and so he cant run away even if he is faster. So everything you can say is: u mad bro?^^ Have fun with this.

Twitch is a more offensive champ but with him you dont stay on youre line, because of ambush. When ambush is on 5 you can be invisible for 50 seconds and that's why i skill ambush all the time. When an enemie has less life but he is on another lane you can run to him invisible and attack him from behind so he cant run away because you can slow and you have poison with twitch :). At 3vs3 i stay top lane with him the whole game and i come down when an enemie has less life. 5vs5 i stay bot lane so i can switch the lanes every time when an enemie has less life.
The long time he can be invisible is why he is one of the most op ad champs in league of legends... Time to own ;)

Karthus is an ap defensiv champ with less cooldowns. With Lay Wayste you can place bombs which deal magic damage to enemies nearby and the cooldown is less then 0.5 seconds. He can sloe too and with Defile he deals surface damage to enemies. This is great in teamfights because there is a bigger chance to get kills and you help your team, because you weak the enemies with this skill. But now i want to tell you about his ultimate. I think it's the most op ultimate from LoL. Requiem deals damage to all enemies no metter where they are. When you die in a teamfight or when you see that enemies escape with less life you can use your ultimate. This is why i love Karthus :P^^.