BS is a hard counter to AM. Nothing much to say. Got my ass handed. Found out the hard way today. Early game safe laning phase he just comes in as a stand in while his support his off ganking elsewhere. My support left me too. So it was just and him until lvl 11. I didn't abandon that lane because no one was helping me gank or defend. First kill he just comes in sees me farming in lane – opens rupture. I fight back but no use. Dead in 4 secs. This continued until he killed me 5 more times. I kept farming. Got battle, manta, linkens by mid game after just farming. That was when the game started to turn around and I was killing. BS made mistakes like going solo and being arrogant. It cost them in team fights. With that build I was still doing very little damage but getting a few kills here and there. Wat radically turned the game was skadi and abyssal. But by then it was so late game I couldn't enjoy the kills anymore. I died a total of 10 times the whole game 6 of which were initiated or claimed by BS. Anti mage is practically useless against him 1v1 even in late game.
We did manage to win the game thanks to good support from skywrath.
Source: https://www.reddit.com/r/DotA2/comments/sqm008/lets_talk_antimage_vs_bs/