My company just moved to an office where the only avialable Mincha Minyan to me does not say Tachanun. They also say Chatzi Kedusha (no Chazarat Hashatz). Is it better to daven with them or alone? thank you.

Obviously what should be done is that a full repetition of he Shmoneh Esreh should be said. (See Shut Yechaveh Daat 3:16). If you can persuade others in a way in which it will be accepted you will be doing a big mitzvah. In regard to Tachanun, in a place where there is no Sefer torah, Tachanun can't be said with the leaning on the arm but should be said without it. (See Rema Orach Chaim 131:b). (Other than Yerushalyim, where the minhag is to say Tachanun even where there is no Sefer torah.) Nonetheless, there are situations in which Shmoneh esreh can be said without repetition; such as when mincha stars just before sunset . (See Shulchan Aruch 232:1). Also there are situations in which people are unfortunately not paying attention to the Chazan and not answerin Amen and the poskim said it is better to not to do Chazarat Hashatz.. (See Shut Yechaveh Daat 3:16). So, if you have no other Minyan, I would say it is better to daven with them.