If you look at the stats on your team compared to the requirements of the mission you can tell what your odds look like. If your active squad is over all 3 of the phys/men/tac recommendations you are guaranteed to succeed. If you meet 2 of 3, you usually succeed but not always(might literally be 2/3 odds). If you meet 1 of 3 you rarely succeed, but sometimes get lucky. If you dont meet any of the numbers you cant succeed. Promotion missions are special - you must hit all 3 goals to succeed. A 2/3 squad never wins promotion. If youre having a hard time getting through a promotion, your best bets are overleveling, swapping classes(the NPCs dont matter aside from appearance, stats are determined by level and class), or changing your basic training to boost the stat thats hurting you most(also, if you havent done all your boost trainings do them for the free stats).