From Galadren's starting position, he should have Total Concealment relative to the wererats in the room since there's a wall there between them.
Thus, he should be able to attempt to enter Stealth as part of his move action, albeit at a -5 penalty to the roll due to moving more than 2 squares.
If he beats the hulking wererat's passive perception, he will have Combat Advantage as the corner should still provide cover enough for Galadren to maintain stealth.
Unlike in 3.X, sneak attack just requires combat advantage - I don't think it requires being able to see your target clearly - and anyway, Galadren's an Elf so his low-light vision should enable him to see the one visible wererat clearly in the dim light.
I will assume, however, that the hulking wererat has cover from Galadren.
Move: Move to C6, diagionally up to H6, and then straight up to C6 (7 squares)
Stealth Attempt (-5 due to full Move):
Rolled 1d20+5 : 17 + 5, total 22
Minor: Mark Hulking Wererat as Hunter's Quarry
Standard: Nimble Strike - After the attack, Galadren shifts back down to D6.
To-Hit:
Rolled 1d20+7 : 19 + 7, total 26
(-2 Cover) (Add +2 for CA if Stealth beats Hulking Wererat's Passive Perception)
Damage:
Rolled 1d10+5 : 8 + 5, total 13
Hunter's Quarry:
Rolled 1d6 : 1, total 1
Sneak Attack (if CA):
Rolled 2d6 : 2, 4, total 6