He doesn't need congress. He replaces RBG with a hardcore conservative, EO's the shit out of the ACA rendering it useless, get's sued all the way to supreme court, 6-3 maybe if roberts votes with them 5-4, and ACA is gone. then over night 20 million lose healthcare. And the rest that have it private, also fucked hard in the butt due to gaurantee's provided by it.
Or alternatively, he just strips little bits away at a time, like he just did recently with transgender folk.