Depends. Just a couple of guys, with laser designators and radios, and a BUFF full of smart bombs overhead, and great things can be accomplished. Ask the Northern Alliance. The Taliban had nearly wiped them out when 9/11 happened. A few days later, they got their couple a guys, and their BUFF, and that turned everything around. The Northern Alliance drove the Taliban clean out of Afghanistan and into hiding over the border in Pakistan within a matter of weeks.
Of course it can be pretty dangerous for the couple of guys. They have to be right up on the front line, where bullets fly, and bad things happen. But it can be fun, especially for guys who like making things go boom.
But you would think just a thin stiffening of American soldiers, and a mob of locals, Peshmerga perhaps, and plenty of air support, and ISIS could become toast. Say a few tank companies to show the way, a plentiful supply of guns and ammunition for the locals, supplies flown in by air, some decent intel from the locals, and a bit of luck and problem solved.
The way the newsies tell it, doing ISIS is as bad as doing WWI trench warfare over again. I don't think it's that hard.
With luck, in 2016, we could elect a president who understands these things.