Double combos back to back also gets him down in three swords. There's very little in it, so I guess it's simply down to sword bearer preference. For absolute efficiency the most mathematically sound combination is heavy attacks all the way, with the odd Super thrown in, but that's certainly not the safest way, as you're cutting it extremely close. I always do two combos, wait for him to be downed again, then two more combos. This will get him down in three swords, which is the same amount as all heavy attacks, only with a little more breathing room.