How to replace just a few boards in hardwood floor
The last board is always the tough one. A trick that usually works is to cut off the lower half of the groove. Now that last board will simply lay down in place. 

The problem is then to get it to stay down. For that you may need a few well placed finish nails, set into the board. Then use some putty to hide the small nail heads. A bead of glue along the tongue may also help to hold it in place.