You can do this with the help of all your units, but there is a chance that you will lose a few soldiers and there is no way to recruit new mercenaries later on. They can't be defeated in time, so the easiest way is to just run to the other side.

A valley in front of you is filled with enemy towers ( 6). When you get to the goblin store, a nasty surprise awaits you. However, all you have to do is buy an amulet in a nearby shop, order Arthas to run through the valley and summon the units on the other side of the valley. The valley filled with many towers of the Undead can be problematic.