This can probably only be improved by about a second or two at most. Besides maybe controlling my units better, the only real way I can think of is to have Arthas level up as early as possible to get the aura speed bonus. Unfortunately that would mean losing the Crypt Lord and then quickly after killing Illidan (and the rest) with the remaining army. This happened once to me but I was using a slightly worse variation of the strategy I used in this video so my time was a bit slower.