The consensus seems to be that Obama is in a box because he has presented himself as being above politics as usual, and doing so prevents him from "going negative" on Clinton, which he needs to do.  I don't think this is a hard box to get out of.
I don't think it would be hard to explain -- even to the press -- that honest, fair, substantive criticism is a good, necessary, and important part of any political debate, and that criticism is only dirty or bad if it is dishonest, unfair, or non-substantive.  Even dull-witted people can be brought to see the difference between fair criticism and mud-slinging.  For years, the press has seemed not to know the difference, treating all criticism of an opponent as uniformly "going negative."  So a candidate making an entirely fair, reasonable criticism is lumped together with an opponent whose criticisms are cheap, dishonest mudslinging.  Obama would do us a great favor by simply explaining this point.
If Obama explains this, he can be as severe in his criticism as he wants to be.  He needs only to make his criticism conspicuously honest, fair, and substantive, and to deliver it in a dignified tone.  There is, needless to say, a great deal of damning criticism to be leveled at Clinton.
Reminds me of a speech by Henry Grattan, a member of the Irish Parliament, in 1800:

HAS the gentleman done? Has he completely done? He was unparliamentary from the beginning to the end of his speech. There was scarce a word he uttered that was not a violation of the privileges of the House; but I did not call him to order—why? because the limited talents of some men render it impossible for them to be severe without being unparliamentary. But before I sit down I shall show him how to be severe and parliamentary at the same time.