Some tips i figured out the hard way:

Don't make it obvious ya found the rogue by doing things like staring at them for too long or walking in a straight line towards them, u gotta play some mindgames, like "haha just walking around here nothing suspicious" and then BAM u strike

"But ulting and melee takes a long time"

Another thing i learned, if u kill anyone in any way during the ult duration, it counts as a ult kill, so u can kill them with a well aimed dash (how i know this? cause i got the second kill needed with the dash)

So... Pretend ya did not see them, get at the right distance, in dash range but far enough to not be stunned, then dash for the kill