@Razh:
I think the question whether Robin could take Trebol alone is pointless, since we all know they won't be fighting one on one. It's not going to be like Skypiea, where everyone was separated. Here the goal is to knock Sugar out, not to beat Trebol. It's entirely possible they will manage to knock Sugar out, but fail to defeat Trebol at this place.
We don't really know that at all.
I dunno if you've noticed but eh…everyone pretty much IS separated, on mainland Dressrosa anyway.
Luffy and Zoro are in the palace, Robin and Ussop are in the port area, Franky's outside the toy house, Kinemon is heading for the toy house etc.
There are 2 Donquixote family members present in the area in question, (Trebol and Sugar) and 2 SH's present (Robin and Usopp).
Since Robin has already been shown interacting with and outsmarting Trebol, combined with the need to keep him away from Sugar (who is pretty much the perfect opponent for Usopp), there's actually a pretty big chance of Robin Vs. Trebol, 1 on 1.
--- Update From New Post Merge ---
@wolfwood:
She's gonna tell him Sugar is in that reinforced barrel over there.
The end.
Ah.
The classic Caribou ruse.
Very sneaky, Robin.