Lieutenant-Colonel Henry Lee had been placed at the head of a flank corps (or “corps of observation”) that consisted of his own legion of infantry and cavalry, some Virginian Continental light infantry, and a corps of riflemen from western North Carolina and Virginia. As briefly noted in Part 1, this corps skirmished with the British on the British march to Guilford Courthouse. Lee’s men then took position on the extreme left end of the first American line.
It so happened that the right end of the British line (Regiment von Bose) passed through this field, giving Lee what he thought was a magnificent opportunity to stymie the British attack. He later wrote:
[the men] raked by their fire the right of the British wing, entirely uncovered… The appearance in this quarter was so favorable that sanguine hopes were entertained by many of the officers, from the manifest advantage possessed, of breaking down the enemy’s right before he approached the fence; and the troops exhibited the appearance of great zeal and alacrity. 
However, he claimed that he could not capitalize on the opportunity, because of the rapid collapse of the North Carolina militia on the first line (see Part 4). The sudden retreat of the North Carolinians “threw the corps of Lee out of combination with the army, and also exposed it to destruction.”
Regiment von Bose, aided by the 1st battalion of Guards (brought up from reserve), began to pursue Lee’s men. However, according to Charles Stedman:
the first battalion of the guards, and the regiment of Bose, [were] greatly impeded in advancing by the excessive thickness of the woods, which rendered their bayonets of little use. The broken corps of the enemy were thereby encouraged to make frequent stands, and to throw in an irregular fire… 
The pursuit of Lee caused the 1st Guards and Hessians to become separated from the rest of the British army. Soon, they would find themselves in a desperate fight for survival in the deep woods.
At the beginning of the battle, Regiment von Bose was placed on the right end of the British line, alongside the 71st Regiment of Foot (Fraser’s Highlanders).
However, after the American first line was defeated, the two regiments headed in slightly different directions, and a gap opened between them.
The further the two regiments advanced, the larger the gap became. In this image, the 71st is approaching Brigadier-General Edward Stevens’ brigade, while Regiment von Bose skirmishes with riflemen in Lee’s corps.
However, Regiment von Bose was not without support; the 1st Battalion of British Guards was soon ordered up to their assistance.
Lee’s Legion (foreground) and Campbell’s Virginia riflemen (background) skirmish with the British and Hessians in the woods.
Parties of riflemen contest the Hessians’ advance.
Virginia light infantry cause trouble for the 1st Guards.
In this post I describe the beginning of what is known as the "separate battle" at Guilford Courthouse. I've commented previously on different ways the source material for this part of the battle can be interpreted (see here).
1. Henry Lee. (1812). Memoirs of the War in the Southern Department of the United States.
2. Charles Stedman. (1794). The History of the Origin, Progress, and Termination of the American War, Vol. 2.