If the enemy's ground battle is as terrifying as their naval battle, then there's nothing more to say—this formation is quite safe for retreat as well. After all, sacrificing himself just to save Gan Ning is the sort of thing Zhou Yu absolutely wouldn't do.
If Kushan's ground battle is still passable, then Zhou Yu can quickly extend both wings to fight a suppressive battle, forcing the Kushan Navy to shift their focus from Gan Ning to the Land Forces, easing Gan Ning's pressure so he can hurry up and run.
Of course, if they've already reached that point and Gan Ning still doesn't retreat, Zhou Yu feels that with his temperament he's already done all that righteousness demands, worthy of Chen Zichuan's trust. Whether Gan Ning runs or not, he himself will pull back first.
If Kushan's ground level is below a certain standard, Zhou Yu plans to trade some manpower, and forcibly use the central formation's "chef's knife" to hack the enemy Central Army to pieces.
