You can "easily" clear Gruul's Lair with 1 Retadin, a full PvP Bear Tank, and a Surv Huntard with a mouse that dies halfway through Gruul's fight and has to change the batteries on the fly (source: personal experience).
The health value for the bosses in Gruul's Lair and Battle for Mount Hyjal is bugged and it's waaaaaay higher then it should be. IDK why they set the value so high, it was the same when they released the instances (so it's not like it ****ed up after an update or something). Anyway as i said you can actually clear Gruul's Lair (even tho it will take an eon), but for Battle for Mount Hyjal i can't tell, i tried to do it with a friend of mine and we wiped at the first boss, not 'cos we died but because the boss destroyed Jaina and she needs to survive the fight otherwise you lose, after the first attempt we said "**** this, we are not going to waste 40mins burning down a 40-50m dummy".
That being said i absolutely discourage you to try these 2 raids unless you are willing to waste a good chunk of your free time, you will most likely need 6-7 people in order to clear the raids in a decent time and still you may randomly wipe in BfMH. You can actually report the health bug on the bug tracker if you want, but my guess is that nobody will care to fix it 'cos that bug has been haunting those instances from the very moment they have been released.