Reflects 80% damage at 10/10 skill
Flame Guard deflects damage
So the captain still takes the damage, right? Can't imagine he'd not 80% is a lot.
Flame Guard does not stack with hero skills causing damage deflection. Such as for example Michael's skill deflection. We can only assume that higher reflect wins. So if you will put FG on him he will reflect damage according to FG, but when he is under "damage reflect" skill, FG is not working until skill wears off. But we need confirmation on that as always. If this is what you mean.
Granted, it is a bit hard to tell exactly, but here is a video with the lil trio in action, maxed. Slingshot Captain does not reflect 100%
Tubys World Video
Name: Rusty (Rustyx in game)
Guild title: resident wisenheimer.