征服

conquer; vanquish; overcome; subjugate; overpower; defeat; master; suppress; conquest; subjugation; vanquishment

难题

difficult problem; tough question; difficulty; load; challenge; complexity; puzzle; ill; embarrassment; travail; brainteaser; teaser; bugger; poser; hang-up; toughie; conundrum; stumper; brain-teaser; dilemma; a can of worms; a tough nut to crack; a monkey on your back; a rough ride; hard nut to crack