Thankfully for everyone, Raiden happens to be in the area for reasons that are largely unimportant to the current situation. Generally, he tends to leave hero fights alone since they tend to have the whole thing under control - which is what he assumes is going on here until he hears Milly shouting at the thing to let her go.

(God that is a gross thing, now that he's actually looking at it.)

In an ideal world, what he'd like to do is take some time to track this... beast. Assess its weaknesses. Take it down without much fuss and without anyone else's safety to consider. He doesn't have the time for that right now though, so what he does instead is he just hurls his sword at the thing. He's not especially concerned with whether the sword actually lands, so much as it should, fingers crossed, distract it enough to make it drop Milly and hopefully divert its attention towards him.

Either way, he's barrelling towards it now. So if it doesn't get distracted by the sword, a punch in the face might work.