I play Karth from time to time, and when I do I play Allahu Akbar Karthus; Sunfire Cape, AP-Life items, semi tanky and just dive into the enemies, you dont have a stun or anything? ALLAHU AKBAR! I get into the middle of the five of them and deal all the damage I can until they kill me afterwards I use my ult or I just spam skills, it doesn't matter, somebody always dies with me and the enemy team looses a lot of life, it has always worked for me, just make your team know that you are not crazy and that they should take advantage of your sacrifice rather than just being cowards that sit behind and watch you die.