WoWDB

A reference to the animated series Captain Planet and the Planeteers. The basic premise of the series was that five teens who face an environmental villain(s) would pool their powers of the elements - and heart - to create Captain Planet, who would always say, "By your powers combined, I am Captain Planet!" He then would proceed to defeat the villain.

[Still unable to edit the prior post, so I'll attach this as a reply.]

I did this last night with Aliok (mage), two guild members (warrior and paladin), and myself (enhance shaman). The area is not unlocked yet, but there is a way to get into it – which is not exploitative, more a simple geographical oversight on Blizzard's part – for those who know how. 

For both of the criteria, no tanks or healers were used.

All four blessings active

This part is easy enough in relative terms. Just aggro all four of the guardians by inflicting a couple hits of white damage on them and then wait until Guardian Tak'u reaches 100 energy, which is when he becomes attackable. The abilities that need to be watched out for:

 Waves of the Hydra A healing DoT that must be removed via spell steal or dispelled.
 Fire of the Serpent - A channeled damage spell that either needs to be healed through or interrupted (preferably the latter option).
 Wrath of the Direhorn - Don't stand in it.
 Lightning Strike - See above.

My group used our various CDs except for Bloodlust (more on that below). We managed to one-shot this criteria, although the warrior and paladin almost bit the dust near the end.

No blessings active

This is the trickier part because while Guardian Ta'ku doesn't hurt quite as much without his blessings, he still has a good bit of hp. Especially with a group because he'll get 50% more health per player that attacks him. With the four of us, his hp went from ~3.5 million to somewhere between 7-10 million.

The reason I mention all this is because once you've killed all the guardians, you will have less than 30 seconds to get this part. So that Bloodlust you didn't use earlier? Now's the time to activate it and go to town! My group had a mix of some LFR, mostly normal, and a couple of heroic pieces from the 5.0 raids, but we cut it a bit close. I wouldn't recommend trying to do this part on a fresh level 90 character.