League of Legends Match History

I need help. I can't escape Platinum 5 for 3 months now. Climbing to Platinum was extremely easy but suddenly I keep getting teams who are toxic or intentionally feed or just decide to engage in horrible fights. The enemy team also usually always has 1 person who carries the other 4 who tend to be toxic or just don't contribute at all.

I've hit Diamond 5 for 3 seasons on my main account, and for whatever reason I just can't do it in Season 6. Anyone have advice? I main Karthus (since Beta) and I play some Zilean support now n then. I need help stopping the out-of-control powerhouse that is the enemy smurf.