if it's only 暗瘡畱下既印 but no more 暗瘡, then i would just use a body scrub 2 times a week. it sill scrub off the dead skin...& make sure you put body cream/lotion afterwards to moisturize & prevent breakout again.
if you have somebody in HK, ask them to help you buy Ettusais whitening body mist... it's something you spray on your back/chest where ever there's acne scar, & it will help lighten up the pigment. it works, i've tried it before... cus in hk, it's so sweaty/humid that it makes me have some break out once in a while... & i'm very picky about keeping my body soft & smooth~ it works really well~