Though keenly aware of each second that passes, you move slowly, exercising what skill at stealth you possess. It really isn't much, and the temptation to delve into Ganondorf's memories for guidance in the matter is very strong - for who is there better to teach you how to be sneaky than the King of Thieves? - but you resist that impulse as well, not wanting to increase your reliance on the Demon King's methods. As it is, you still catch flickers of "inspiration" from your past self - little details of posture, pace, and positioning experienced in past dreams, which didn't make much sense at the time but come back to you now. This is how you step to minimize the sound of your footfalls. That is how you move with your mark, to remain in his blind spot. Here is the path you want to follow, for the best combination of speed and stealth.
Gained Stealth F+
In this particular field of physical activity, your size becomes a hindrance. Even the presence of your modified invisibility spell can't truly account for it; going unseen and somewhat unheard and unsmelled is all well and good, but if William manages to lay hands on you anyway, that will be it. From the way his nostrils keep flaring, it would seem that the young Mr. Marsh has inherited something of a true troll's keen senses - that, at least, is one area where your non-detection spell can cover for you. It also helps that you're in a public bathroom, even the cleanest example of which should have a number of conflicting (and rather unpleasant) scents to confuse William's nose.
You make it past one of William's grasping hands clean, but not the other. While it does not actually catch you, it comes close enough that you can vaguely sense the wind of its passage - and so, in turn, can William sense you.
"Got ya now, Mister Magic Trick!" the ugly boy bellows behind you. You hear him moving, and can picture exactly what he's about to do: make a break for the now-closed bathroom door. If he gets there first, he can block your escape, and if he happens to run over you in the process - not an impossibility in the relative confines of the bathroom - so much the better as far as he's concerned.
Good thing you were prepared for that, and started running as soon as William narrowly missed grabbing you. You make it to the door before the half-troll's taken more than two steps in that direction, and you have it open before he's completed his third. Then you're out, running full-tilt down the hall, William following in your wake and bellowing fit to raise the dead in that language of clashing rocks.
You leave the side-hall where the bathrooms are located and re-enter the main corridor that leads into the arena proper. There, standing in the middle of the passage, is Othrym, assuming a stance not unlike the one William was using a moment earlier. Unlike the half-troll, however, this jotunn-spawn has a few more options for catching his targets. Cold air and specks of snow are billowing out from Othrym in all directions as large masses of ice form on the walls, floor, and ceiling with impossible speed. Already, there is a good half a foot of ice clogging each side of the hall, leaving a slippery space that Othrym can easily guard.