Another option you could take is to raise their level in terms of offensive capability and lower their level in defensive capability. So you'll get monsters with a little more damage and who hit a little more often, but they also can't take quite as much and are hit more often. Maybe a +2 levels for damage and to hit and -2 levels for defenses and hp? I'm sure experimenting will find a comfortable and appropriate level.

You'll still speed up combat a little bit but not at the cost of disproportionate offensive damage. And the monsters will still be able to stick around long enough to do something cool and interesting.