How to lay last row of Hardwood Flooring under an undercut the Baseboard?
Ok I'll keep guessing.  

If this is nail down with no tongue and groove then you slide the piece in and then use a glazer's bar to pry it back out while you nail it.  

If this is tongue and groove nail down or glue down then 


quarter round 
or micro-round 
or cut the tongue off the last piece and glue to avoid face nail.
or cut the tongue off, create a biscuit, glue the biscuit and pry the last piece back onto the biscuit and glue this piece down to avoid face nail.


If you don't want 1/4 or micro round your option seems to be either 3 or 4.