I find myself disagreeing with the advice we are given.
On Normal I just killed the brainwhale at the first ambush site. I had no cruisers built at the time we were told it would be to late if we didn't already have Artemises ready to go--and the force that took it out had no Artemises in it. It was pure Nemesis cruisers. I vectored them to the NW corner of the ambush site. Two of them were in position to fire by the time the brainwhale came into range. I then sent the ones still on their way to the NE corner--and when the brainwhale went out of range I sent the two that had been hitting it over to the NE corner also. It blew when it was just passing straight north from that NE corner.
I do not believe I could have done nearly as well with Artemises.