Maintaining track ident on one primary contact in the LTMA. A contact on a standard route.
Not exactly difficult even if you are fairly busy. Of course to make it easier you would hope that the controller would split the sector if it was possible, if workload dictated.
You have all manner of visual cues (trail dots etc) as well as the ability to tell the pilot to report passing points if you really need to.
Keeping track ident on several non transponding GA aircraft that are just milling around the countryside, amongst a load of other non transponders... now that's more challenging - I'm glad those days are behind me!