Without the full context I can't be sure, but I assume the he in this sentence was exchanging words with other people, perhaps politicians or leaders in a court or some sort of forum.
To say he took on targets means that he levelled some criticism at and/or insulted some groups (such as the judicial establishment and the Revolutionary Guards). To say that these targets were once seen as all but untouchable it means that the targets were so revered, respected, feared etc. that to challenge them or speak ill of them was something that wasn't even considered before this person came along. That other people thought they were too big or too important or too scary to say anything bad about or question.
With further context I can be more specific; there isn't enough context supplied to be exact.