In the comic book, not the movie.
BQ: Who is his arch nemesis?
Iron Man’s weakness is alcohol! His chest plate also contained a pace-maker because shrapnel stopped his heart.
Iron Man’s arch nemesis is “The Mandarin”. A picture of him can be found here:
Alcoholism(as stated in the comic Demon in the Bottle)
He has very strong convictions that have caused conflicts with allies (Armor Wars and now Civil War).
Several women have held great influence over him (Sunset Bane, Madame Masque, etc).
His dedication to Stark Enterprises has caused problems (innocents in the line of fire)
BQ: Mandarin is arch nemesis
As Tony Stark his weakness was and may still be alcohol and maybe woman.
As Iron Man it would be his reliance on his armour (and having it shut down against enemies that can control machinary) or as was mentioned against magnetism ( villians like Magneto) or beings on a greater power level. (mystic or cosmic maybe).
His arch-enemy is The Mandarin
Himself (or Iron Monger)
weakness, sure his chest but not really, an emp is his biggest weakness, that and just not wearing the suit. i guess his biggest enemy, would also be his greatest ally captain america, who for a shortwhile, was pitted against iron man because of different political beliefs, which ended with them going bareknuckle blow to blow with there own seperate teams of heroes. if you mesan generic enemy aim i guess i dunno.
weakness: His chest plate