Wait for opponents attack. Parry, duck or jump to avoid it. Press 4 for quick thrust forwards afterwards. Rince and repeat.
If the opponent is really quick he will be able to come back from a thrust in time to parry, in which case you can choose to only counter when he's doing low or high blow. If he's so quick that he gets back to dodge those too, you're basically screwed.
It won't make the combat as short as possible, but you should be steadily winning on all difficulty levels. If you notice a really slow opponent you can start doing high or low blows to win a bit faster, but if there's any doubt, don't do it. If opponent parries and counterblows, he may get advantage too and get so fast that you can't stop him.
Works with either sword. I prefer long sword as a balanced sword that has a decent chance to defend in time, and still hopefully able to counter before opponent recovers, but all swords should work ok.