[Why, he's so creative we're even doing a combination: picking Ciel up (and getting blood on his clothes sorry) and putting him down a second later, further back on the bridge, away from the attacking monster-- this one, at least.
And then, yes, Sebastian leaves him to survive on his own (please try) for a minute while he turns his attention back to the monster. With only one good arm and no throwing cutlery at his disposal (it seemed a good idea at the time to take off his uniform jacket so as to not get it wet) he goes for the obvious option of kicking the creature in its jaw. Brb while he kills this thing-- or injure it and add even more blood to the water, to start with.]