2 of 2 clarify I think

The Rivevos Ephraim chelek 5:584:3 writes that, since if one misses one word of krias megila they are not yotzei (Mishna Brurah 690:5), accordingly it seems that certainly by parshas zachor which is a Torah obligation if one misses one word he is not yotzei: like the Sefer Mikraei Kodesh siman 7 writes, it isn't any lower than megillah. Therefore one should be careful to listen to every single word.