Interesting question. imo Dragon Tail is a better option however.
Serperior can probably take a neutral hit relatively easily due to it's decent defences, meaning the recoil from Giga Impact isn't as hindering. However, Normal moves provide no coverage whatsoever other than hitting the opponent neutrally, meaning a powerful but non-STAB Normal move isn't as effective as possible. Dragon Tail is great for getting rid of the opponent's Pokemon for a while, though it's -6 priority means Serperior's good Speed is put to waste. However it is a better move than Giga Impact due to coverage reasons and effect, and ingame Dragon Tail beats Giga Impact.