Balrog,
I am going to take an unusually strong stance on this one.
You are worrying too much about your players. One thing my players teach me over and over is that I underestimate them. This last campaign, I even threw and ogre at 3 1st level PC's. They killed it without going negative. They were willing to take the risks and rely on a good dose of luck. Wacky!
Anyway, your PC's have a lot at their disposal in the way of resources. If they want to go in and take on the bad guys, let them. It's not your job to stop them. You job is to provide a fun, dramatic battle if they do that. Some of the PC's may die. If they are particularly ill-prepared, or running through a string of really bad luck, then it could become a TPK, but they might also surprise you on how effectively they can retreat if they want.
Don't let them get into the "DM would never throw something that we can't beat at us" mentallity. That mentallity creates a cushion that they will begin to rely on and the fear of death will begin to recede. Death needs to be a real possibility and when it is, you will see your players becoming more creative and better educated on what their PC's can do.
If they do go into this battle, despite your misgivings, have fun! When the Hezrou takes out the front line fighter, don't have the fighter go down. Have the Hezrou pick him up with those claws, clamp his jaws on the shoulder and rip the fighter apart. Make it cinematic and emphasize the sheer ferociousness of the NPCs.
A slight hijack on making combat dramatic - Roll the attack/damage, and then describe it. Don't look at the above example and think: Well, that would be a grapple and then next round he could rip him apart. Think: The fighter is dead from damage anyway. Did he just get swiped with a claw or can I make it cool and dramatic. My point is that once the PC's are our of HP, make the death dramatic so they can feel that masochistic glee. By the same token, do it with NPC's too. When the fighter finally takes out the Ogre Mage, it's not a hit and he dies thing. See if you can get the player to describe the attack, if not, then do it for him. "Fred the Fighter barely ducks under the Ogre Mage's sword. Seeing an opening, he lunges forward, then up, underneath the ribcage into the black heart of the Ogre Mage. Blood runs down his blade as Fred braces his foot on the Ogre Mage's chest and pulls the sword out. With a final kick, Fred pushes the Ogre Mage to one side, it's tongue lolling out of it's mouth, before he looks for the next combatant." OK, Randy the Rogue, what are you doing? As I said, once the combatant is dead, everything you do is cinematic. Make it fun.