Infernal Ambassadors

Warning : if you plan to get this achiev with a level 100 character: as you one-shot these mobs nowadays, all criteria are just automatically won, except for the Galenges part. If you kill him as soon as he pops, it won't get validated. The achiev says "kill him before he is weakened a *second time*". i.e., he has to get weakened at least once.

So you must wait that the Spirit of Malorne who joins the fighht, debuffs him once ; it takes 5-10 secs or so, just stand there and don't DPS. When the debuff has been applied, then you can proceed on one-shotting Galenges and you will get the achiev criteria validated.

The mobs you must kill for this achievement are the ones fought in the random two-part daily within the Hyjal zone.

Nemesis - (Punting Season / Echoes of Nemesis) - When Nemesis begins to erupt, run over to Tooga and hide under his protective shell.

Millagazor - (Call the Flock / Wings Aflame) - His special attacks will place patches of fire under you. Just keep moving as to not get popped in the air by them.

Lylagar - (Perfecting Your Howl / The Call of the Pack) - Simply move out of the way of his breath when he casts it.

Pyrachnis - (Those Bears Up There / Nature's Blessing) - Only use the Emerald provided to summon him, never to cleanse the stacking poison his attacks apply. Kill him quickly and it won't become a problem.

Galenges - (Between the Trees / The Power of Malorne) - After summoning him with the staff provided, a spirit of Malorne may appear and greatly weaken Galenges. All this one requires is heavy DPS so he dies before another Spirit comes.

One note about Galenges (might be helpful):
If you kill him during first weaken phase, you won't get credit for it. Stop dps and finish him after he grow in size again. Took 3 or 4 summons to find it out.