5 frozen hearts, wheres your precious autoattack now?
6 derpcaps. Resulting in 3.75 times increased AP, assuming they would stack with themselves.
6 thornmails, sit under tower.
6 Banshees, shield every 3,7 seconds. Assuming you actually get through all of them.
Now a teamcomp, 4 banshee busters and Karthus.
Everyone cept Karthus stacks WOTAs, Karthus stacks derpcaps. Everyone dies to ult.
Actually... it'd be more like 4.8x time AP with 6 caps (1.3^6, since the bonuses stack). And since each cap has 140 AP, that's 840 AP... Equals 4054 AP. Karthus ults, everyone dies... Except 6 stacking Banshee's Veil. Though, anyone with AP granting auras will stay with Karthus at base and laugh with him. Let's see, with all WotAs, that's 4 * 6 * 30 = 720 + 840 = 1560 * 1.3^6 = 7529 AP means 550 + .6 *7529 = 5067 magic damage. So, everyone dies unless you have GA or Banshee's Veil. Though, I suppose if the ult fails, there's the 1992 magic damage per second he does next to everyone (which, by the way, does 600% spell vamp, for a total of 11952/3 = 3984 damage healed per second (though, MR reduces this...) So, assuming they manage to stay out that aura range, then Karthus can bomb for 4757 damage to one target or 2378 damage to multiple targets. Every .5 seconds.
... So... yeah... He also heals 6x the damage from each of those. That will hurt.
Vladimir stacking spellvamp.
Wait a minute...
Gangplank stacking Infinity Edges. One shot's all he needs.
The passive itself only makes criticals do 250% damage. So, that wouldn't stack. If it was reworded... Crits would do 762% damage. Which is probably enough.