Generally (I don't know your exact model) it could be loose in two places
- the insert into the strut (if it has an insert), and this has a nut on
top of the insert housing. If it is the top bearing, there's usually some
means - hex or socket - on the rod to hold it while you tighten the nut.
Of course it could be a failed top bearing or the housing for that could
be loose.

Hello:)   I had this problem about 2 years ago on the passenger side and
took it in to the mechanic.  I remember him taking a large pair of
channel lock pliers to hold something under the fender while he
tightened the top strut nut.  My mechanic is now gone and I peaked under
the fender and dont see where could of put it to stop the strut from
spinning while tightening?  I dont want to put it on the chrome rod
unless that how its done? I put the car in the air and grabbed the tire
at 9 and 3 'clock and can see the whole assembly move.  Strut mounts
were new about 2 years ago.  I was hearing a noise over bumps on the
left side that why I went searching for the problem.  Thank :)Nate