I can't get on with treating in a larger pen as there's always something to catch or knock you or fall over, and the OP will be in a field so it will quickly turn to slippy mud and muck with all the hooves in the same small area. I have to let three or four out at a time from the crowded catching pen into a smaller handling pen to do whatever needs done, and thereafter back to the field. Keeps the bulk of them calmer until their turn, I can see more and it gives more room for me to work.
All about finding little tricks and methods of breaking the flock down into manageable units. Everyone will have their own ways of processing the sheep.