If she suspects that the man standing in front of her holding a spear is not only trying to hurt her, but is also holding back, the instant he thrusts his sword into the ground [you didn't even tell me that part] her shield is up.
And her shield (and Sailor Moon) is penetrated by a spear leaving her dead.
And did he survive? I read on the Shaman King wiki that he can regenerate.
The attack? Yes. Oversouls can be regenerated with Furyoku.
In that case you don't think she'd know a bit about him and his abilities? That would put her even more on edge.
And despite how on edge she is, she gets penetrated by a spear.
Sailor Moon took a planet destroying attack with no defense, was injured, but still had enough stamina and power left to create a shield that not only stopped a charging Galaxia [someone who rivals her power while healthy], but her soul snatching attack as well.
Good for her. She still gets stabbed with a spear coming out of her brain via every major organ system in the body. Something that would kill her in short order.
If she survives and Ren tries a counterattack all Sailor Moon has to do is shout "Sailor Moon Crystal Power" and it would wipe him out, unless he's capable of using Oversoul to protect himself instantly.
If Sailor Moon survives the initial wound (which is questionable) her top priority isn't going to be Ren. It's going to be not dying. She's going to have a tough time saying anything after an oversouled spear passes through her entire body, after all. Shishen Tougen Kyou is just to put an overkilled end to an impaled Sailor Moon.