Hitting three or so people with a 300 damage Spinning Slash, plus a 300 damage Shiv proc on three people. The Shiv proc and Spinning Slash are likely to be used a second time, but only on one or two targets. 2400 total. Plus, let’s say half a dozen crits for 400 each. 5400 damage being boosted between autos and Shiv/spin. Trynd has probably five seconds between the time he starts hitting someone and the time his ult runs out.
Karthus has probably 10.5 seconds between the time he starts hitting someone and the time his passive runs out, with Zhonyas. Requiem hitting five people for 600 damage each. 3000. Each tick of Defile does another 150. Let’s say this hits two champions five times each, and another two champions twice. 2100. Let’s say he also lands two single-target Lay Wastes, and two double-target hits, for 600 and 300 damage respectively. 1800. This is a total of 6900 damage, more than 25% higher than Tryndamere’s output. Neither Wall of Pain nor Conqueror are factored.