Garnet might be physically stronger but I doubt she is faster or more agile. Opal had excellent strength and mobility in Giant Woman (punching through giant bird, jumping across chasm, graceful & rapid dodging and movement) to the point where I think she could outmanuever Garnet. Opal keeps her distance and rains down arrows for the win. 

It would be close especially since Garnet is so durable and has future vision. Opal would have to find a way to stay fused. 

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