Someone had this problem on the Steam forums too so I took a look, I'll just copy paste here:
huh, I gave this a try for you and failed the first time too. If you wait for all the dialogue to finish at normal speed your Nexus will be destroyed.
Got it on the 2nd try though
You have to spam click on the dialogue so it skips through at a faster pace. Than grab the Promethius and attack asap. He will reach the enemy Dread and the first or 2nd salvo will force the that dread to turn and face you. You'll kill it and you'll be on your way.
That Dread received a significant boost in the last update patch or two so it now kills that Nexus faster than it would have when they put this level together. Hopefully they tweak it a bit now (esp. as Generals is in this thread).