You remove the archers. Then you follow the
FAQ (part 2):
Q. As characters are not normally allowed to form
a unit together, how are we to resolve a situation
when two or more characters have the unit they
were attached to destroyed around them? Do the
characters remain a unit or do they separate?
A. They remain a unit until their next Movement
phase, when they must separate if able to do so (i.e.
if they are not fleeing, rallying or subject to another
form of compulsory movement). Note that, if they
are fleeing, they are allowed to take a single rallying
test as normal, as they are still a single unit.