archfoe (noun) archenemy: enemy
Sinonime: abreast, acquainted, apprised, at home with, au courant, au fait, aware, cognizant, conscious, conversant, grounded*, in on, in the know, informed, introduced, kept posted, mindful, no stranger to, plugged in, savvy*, tuned in, up on, up*,
versed, versed in, well up in, with it