Actually, Shura has another idea, which is that the reverse array might not work against the two bosses, the Gray Mist and the Sanctuary, but it might work against the Corpse King, after all, the Corpse King is a zombie.
As long as the Corpse King is dealt with, even if Gu Shanhai can't join the battle, the two bosses can still have a chance of winning, even if the odds aren't high.
That's still better than doing nothing.
The main disadvantage lies with the Grand Heaven position; Shura has Gu Shanhai's equipment, so when facing a boss like the Sanctuary, even if he can't win, he can still protect himself. On the other hand, the Grand Heaven position, even with Tai Sui Gu and Cui Yingying in tow, pales in comparison to the Gray Mist.
