As others have said, I think the emphasis on group vs group combat is because a fight between a group of PCs and a single creature often favors the PCs because they get multiple actions for every one that their opponent makes. For a single tough monster, it looks like the solution will be to give it more actions.
I have previously speculated that this may be an indication of how monster statistics may change when the monster adopts different roles in 4e. The giant that the PCs must work together as a group to overcome at 5th level might have the statistics of a Solitary Bad Guy (including, possibly, the ability to attack multiple opponents with a standard action swing of his club, dealing moderate damage to each opponent), while the giant gang that the PCs encounter at 10th level might have Grouped Bruiser stats and be limited to a single attack per standard action (but one that would do hefty damage).
However, I would be wary of the potential downsides of monsters being able to do too many things before the party can react, such as the possibility of sending one or more PCs from full hit points into the negatives in a single turn's worth of actions. In 3e, this has happened a couple of times with a DM who liked to make a single initiative roll for his monsters, and liked sending them against the party in big groups. One way to ensure this is to allow the monster to make multiple reactions, i.e. multiple immediate actions in response to the PCs' actions, instead of making multiple free actions on its turn.
