Board Thread:Manga/@comment-26506584-20160426174724/@comment-86.40.29.37-20160428235002

At the moment, the shifter that can be sacrificed most easily to faciliatate the use of the serum by a Survey Corps member seems to be the Beast Titan, as he is much less developed as a character than either Reiner or Bertoldt. That he was claimed to be "the strongest titan" and has previously defeated Reiner as the Armoured Titan with relative ease, and his rank as a war chief, shows that defeating him would obviously be incredibly difficult, even with Levi and Erwin's current tactic.

Due to the dramatic nature of acquiring the serum in Kenny's dying moments, it is an example of Cekhov's gun, so must be used sooner or later. The present story arc is effectively the culmination of most of the series since the Battle of Trost, so using the serum to find out what is in the basement seems an appropriate use, especally considering the current hopeless situation.

As mentioned above, it seems implausible for Hange to have the serum. Perosnally, I would argue that Erwin will use it. Levi is already humanity's strongest warrior, so making him a titan shifter would seem like a waste of his human abilities and make him, quite simply, overpowered. Eren needs to have an edge over other, more powerful human characters like MIkasa, so he must be the central shifter.

Meanwhile, Erwin could use the serum to further distract the Beast TItan by trasforming after their suicidal charge and his apparently unavoidable death. Though Levi possesses the serum, I feel confident that he would have given it to Erwin before their charge if requested. Similar small, but vital, preparations have been only revealed retrospectively before, as in the Special Target Capture Devices and Thunder Spears. Isayama has, in gerneral, shied away from killing any character so significant as Erwin (disregarding Hange's probable death until confirmation) so this may be no different. Erwin would also be able to fulfill his dream of seeing the basement if he could defeat the Beast Titan, for example. If we consider the theory that the Beast titan is Erwin's father, it may add some extra pathos to the event. If not, it would be an intriguing fight nonetheless.

Whoever uses the serum still faces the problem of not turning on nearby humans. Eren barely controlled his titan originally, though he was already a shifter from consuming his father. We could always assume that Erwin's or even Levi's immense determination would grant them some control over an otherwise mindless titan, like Eren's way of controlling his Rogue Titan  in the Battle of Trost simply by focusing on avenging his squad and killing all titans, of course.