In a first attempt I was thinking that techallem is from the radical Klima כלימה as in Ahava rabba lo nikalem .לא ניכלם And here in Birkat Haminim it is not plural asתמגר ותשבר aren't. Techallem is to make shame. An example of this radical is in Bamidbar 12.14. but it's probably not right because of the comment of @ba.
But now I just have read the Abudarham about this Beracha, which was lacking in previous editions, probably because of census. In the new editions, based on a manuscript. He says that Kaleh, this is his Girsa, without Mem, is exterminate. He said that the end is Teshaber Vesachnia because it's similar to the Chatima, shover Oyvim Umachnia Zedim, and the part before the Chatima needs to be similar to the Chatima.