also ahri is the best c;
she's totes my fave to play.

Be more aggressive. Don't use a sentinel, as most attacks hit your whole party anyway. So long as you can survive each burst, you should be fine. Just keep a Combat Clinic paradigm easy to switch to. Go Ravager, Saboteur, Medic(Light, Fang, Hope, is best imo), most of the time, throwing in a Ravager Ravager Ravager or Rav Sab Rav, to get that chain up. When you stagger, go full Com Com Rav, or Com Rav Med, if you need to heal while attacking. Also, make sure you have a Rav/Med Sab/Sen Synergist paradigm. Any one of those combinations is fine, as long as you like it. Keep your buffs up, and keep debuffs on him. In case you didn't know, Saboteur debuffs slow the stagger bar the same way Commandos do, so you don't need to have a Commando when building up the chain. Keep hammering away, and if you still hit the doom meter, when it's almost out, use a summon at the last second and try to finish him off with that, since it pauses the Doom counter.

you're playing league?
add meh: rynvie
I rarely get on any more, but yeah. League is cool.