Serenity isn't the worst for that fight. In terms of raw dps output, Chi Explosion will outperform Serenity assuming you keep the add w/clone up for a hefty amount of time. The way our guild does it, no one even touches the add, and we just let passive cleave kill it. More times than not we'll be transitioning into the next phase with the pre-existing add still up. We found as long as the previous add died before the new one spawned, we'd be ok.
However, if your group is focusing too much on the add, and then you're finding yourself just doing single target with Chi Explosion on boss, than Serenity might be the smarter talent choice.
I also really like Serenity as it compliments Phase 3 really well. The damage debuff you get scales exponentially the lower the boss' health gets. If you time your Serenity's right, you can get your last one off right before the boss dies and absurdly pad your meters.
If you DO chose to run Chi Explosion, you need to inform your guild that it is not the spec for single target. Assuming the two adds are stacked, you will do MORE damage to your focused target with a clone out as opposed to single target. Chi Explosion scales exponentially the more targets we can SEF (Which is why fight like Iron Maidens we can rank ~20k dps above everyone else.)