Keep in mind that it isn't the strawhats that are trying to take down Sugar, but the dwarves are, together with Kyros (although he wouldn't stick around to deal with Sugar as his main goal is surprise-killing Doflamingo). Robin and Usopp just got mixed up in the situation, but they aren't leading the operation. Additionally Robin and Usopp were never the fighting types to begin with. If Luffy would have been there, it would have been a totally different situation. To this date (chapter 787), there is no other Straw Hat that even knows about Sugar and her ability.
Additionally the dwarves are known as great cultivators as they stated that they can cultivate any plant, as evidenced by the abundance of plant life on Green Bit (wiki). Therefore it makes more sense to take down their enemy with spices than by force. It's their speciality and what they are famous for, despite their inhumane strength. (chapter 726)
Lastly, this is probably the main reason, without them actually ever stating this obvious fact. They know they aren't strong enough to take Trebol down. So a frontal attack would never work as they know Trebol will always be by Sugar's side and unless they go about sneaking around with spices they would suffer a horrible defeat. In the end, this was exactly how it went down.
Additionally as pointed out by Wouter, even if the Tontatta would be physically stronger than Sugar, they need to take her down, before she can touch them, or they would be turned into a toy, exactly as what happened in the end. They underestimated her, because she was only a little girl. So without using ranged attacks, it would be unwise to attack her. I'm not completely sure why Usopp did not just snipe her though (at that time). He would have been the perfect match as we saw later in the story.
