[@Savo]Well, one thing I was thinking of is that it would be harder to hit her targets in CQC because, for one, less time to line up the shot, and much less room for error. Remember that her shotgun fires slug rounds, so while it hits like a truck, if your aim is off, then you don't have the spread to make up for it. So while she wouldn't have much trouble against slower, tankier targets(though in that case, she'd probably run anyway) faster, more offensive foes will completely overwhelm her if they get close enough and she has no back-up to help her out, as she'd have to due some really risky maneuvers to ensure a hit. Basically, going 1v1 is a bad idea because her opponent can either match or exceed her speed, or take her attacks and keep on trucking with little worry. This is less a problem with Grim, I'd think, since they don't really avoid attacks that much and go down pretty easy if they're the lesser versions.

Is that a good explanation/reason?

Edit: Also, I forgot to put down the types of Dust she uses for the shotgun, so I'll put them down here as:Gravity, Fire, Ice, and Electricity