When you realize that if Shulk had beaten Zanza any sooner than he did, Rex's world would have been destroyed.

Essentially, the events of Xenoblade Chronicles 1 and 2 take place at the very same time except in different in different dimensions.When Klaus destroyed/created the universes, half of him (which was evil) went to the dimension where Xenoblade 1 took place while his good side stayed in the original universe which is that of Xenoblade 2. What's more, if one half from either universe dies, the other one dies as well and his death will cause the loss of Rex's powers so he is basically on a race against Shulk on killing the big bad of their universes.