So if I've done my math right.
It takes 18 big swings to kill Crota.
Someone showed that 1 big swing is roughly 15K damage. So lets say Crota has 270,000hp.
That means that a Weapons of Light swing should do 18,750 per big swing, or 15 total swings to kill Crota.
With the extra perk, a big swing should do 20,250 per big swing, or 14 total swings.
Now, If you really push for time, you can get in 9 big swings per sword, if using the 3 kneels per. Thus, you should be able to kill Crota on the second sword, with the 14th swing. 13th swing if using the super for the last hit.
What's nice is it should still trigger enrage at a safe time.
Edit: Even if you didn't want to do 3 kneels per sword, you could probably do 2 kneels per swing as long as you use the super once per sword.