That's the problem... with Luden's it IS mathematically efficient. (My first few runs I only added a Ludens and Deathcap to an AD build).
- His ult is a 1:1 AP Ratio, opening the fray. This builds, instantly, 3 stacks of blight.
- His E or Q then explode all three stacks on whoever it clips. (In this instance, 19%x3, or 57%, of Max HP as Magic Damage)
- Auto Attack proc for Lich bane.
- Someone is dead, in this instance even Alistair was gone in the opening Salvo, generally catching 2-3 with it (instant deaths). Varus' passive procs.
- Pop whoever is stupid enough to still be standing there.
- Q or E are both good choices to finish this Quadra or Penta.
Incidentally, with his ratios and scaling, both AD or AP are efficient as far as math, as long as play style lines up. In this, he's like Ezreal - People play AD Ezreal because of his passive - Varus can do more damage with his ult and Blight than he can with his Q, however he loses the benefit of his passive.
So it's either:
Q+Passive
or
Ult+W
Both are equally lucrative.