Tried doing something similar on the PBE. Was against a Kog'Maw with an identical build.
I went Blade of the Ruined King + Hurricane, didn't have much of a plan beyond that point (could have gone AP afterwards, but I wanted the sustain from the blade). We lost, mostly because top apparently fed a Fiora who was 12-0 before she went down to bottom lane for a visit.
Never really got to see if Varus or Kog'Maw was effective with that build. Varus has the small bonus of being able to root people in place which can help hitting as many people at once as possible, but if they spread out you're worse off.
Just did it live in a normal game. Got an ult off into the group, auto, auto, auto, detonate for 25% of their health. It was glorious. --- IGN: Sabotron "Just imagine if Hitler had gone Paragon instead of Renegade!" WolfDJengo