If MS come up with significant technicalities or challenges to the amount due, the IRS will have to choose between litigating each one of them, which both costs them money and involves the risk of losing in court, or coming to an out-of-court agreement with MS.
Obviously if MS don't have anything that won't get laughed out of court, they shouldn't get much of a reduction. If your challenge to your tax bill doesn't involve anything where the rules are hard to interpret and precedent has not been set, you also won't. (If it does, your tax affairs are probably much more complicated than the average citizen. Which is obviously true for Microsoft.)
Worth bearing in mind too that the number might be an optimistic headline figure which has been rounded up in various ways by the IRS, to make themselves look tough, and to encourage MS to settle.
The IRS settles for lower amounts ALL the time, it wouldn't be even remotely exceptional if MS gets a discount. They have an entire department to handle these offers in compromise: https://www.irs.gov/payments/offer-in-compromise
Even with 30B cash in the bank, MS will do everything it can to lower that bill. First they will fight it, and take all the little wins they can get. Then whenever it's pretty clear what parts they can't win, they will then start negotiating with the IRS to try and pay 50 cents on the dollar or something.
Part of the tactic is delaying, the longer they can put off actually writing the check, the better, as money today is worth more than money tomorrow(inflation and time value of money).
Obviously if MS don't have anything that won't get laughed out of court, they shouldn't get much of a reduction. If your challenge to your tax bill doesn't involve anything where the rules are hard to interpret and precedent has not been set, you also won't. (If it does, your tax affairs are probably much more complicated than the average citizen. Which is obviously true for Microsoft.)
Worth bearing in mind too that the number might be an optimistic headline figure which has been rounded up in various ways by the IRS, to make themselves look tough, and to encourage MS to settle.