R' Slifkin recently started giving shiurim at the shul he davens in about topics related to the animal kingdom (not science and torah). The "Rav Av Beis Din of RBS", R' Perlstein recently publicized an anti-Slifkin letter denouncing these shiurim.
The letter is written in flowery and rabbinic Hebrew and is quite hard to literally translate. Here is my attempt at a translation:
Regarding what you asked about Natan Slifkin, it is definitely prohibited to participate in the shiurim of one who distorts and cuts up the words of Chazal. Woe to the ears of one who hears such nonsense, especially since all the Great Sages in both Israel and America have declared that it is prohibited to read his books and to listen to his shiurim, and it has already been publicized many times, who would not be careful with his soul and keep their distance from him. Give it some thought, when expert doctors warn against eating something specific because of a small possible danger, who would dare eat it, and this all the more so that the Great Sages said with certainty that his writings are prohibited and his words are suspected of containing heresy, who would want to lose his world (to come), destroy his portion and abandon the source of pure spring water (e.g. Torah), and drink from broken wells, at a time when the torch that was given to Moshe is passed through the generations via our sages, who kill themselves in the tents of torah and guard the holy watch, they are the same ones who said not to touch (Slifkin's works) therefore I say faithfully, (quote from a pasuk) people should listen to shiurim only from those who are confirmed in Torah and fear of Heaven and the words of chazal are embedded in their hearts like nails. With this you will be blessed with children and grandchildren learning torah and doing mitzvos from the purity of their heart as it has been passed on from generation to generation.
We should continue to go with the light of the torch of the Great Sages and blessed are we that the Torah was given to his guards.
Rafi Goldmier published a response from R' Malinowitz who is the Rav of the shul that R' Slifkin davens in and is the general editor of the Artscroll gemaras.
The response is quite interesting, R' Malinowitz accuses someone of forging the letter as he says that the letter is so inaccurate that it could not possibly have been written by R' Perlstein. For a full translation see And the Slifkin Saga Continues...