Personally, while duoing with a caster, I like to get my opener in, wait for at least 80 energy, then get a quick maim > shred between casts. But that is just me, and I don't think you have maim at 61 (you get it at...62? 64?). Really, just mangle until it is at 35% or so and then FB if you have 4 or 5 combo points. Sometimes, depending on how good your gear is, a 3-point fb will be less damage than a mangle, and it uses all of your energy on the side. Because of the using all of your energy part, I rarely use fb any more. Even though it is slower (I think, haven't actually timed it) to maim > shred, I find myself using that on anything that can be incapacitated.