I was going to say that 12th level characters would mop the floor with hill giants, but you already discovered that. One thing you could do to improve the giant´s damage output/staying power of the classes giants is giving them longspears and quickdraw. If they are charged, they have guaranteed an AoO and, in the giant´s round, a full attack. If the giants charge, you stab the PC at 15 feet, preferably using some good power attack. In his round the PC can´t full attack you, since you´re at 15 feet unless he has a reach weapon (and if he has, you just attack at 20), and they are in your threatened area, so they may be opportunity attacked if they move more than 5 feet. In your next round, you just attack with the spear (if the PC is at 20-25 feet) or quickdraw and full attack with other melee weapon (if they are closer)