Ninja always strike first, but samurai always strike last. In other words, ninja have initiative, but samurai are allowed one final attack, even if their stamina has been depleted. This makes it very difficult for a ninja to defeat a samurai and make a clean escape. The only safe way to prevent the samurai from landing a final blow is to attack and then step out of range (the extra point required to step out of range means you must finish the samurai with a weak attack, and even then, the samurai can attempt to pursue you). In short, being a ninja is difficult; they must escape in order to succeed, whereas samurai must simply defeat their opponent, regardless of what happens to them.
Paying for an action point is a risky choice; by doing so, you've given your opponent two actions on his upcoming turn. If he chooses to likewise pay, he'll end up with three actions and be able to attack you with a Triple attack, potentially knocking you out. There are several ways to deal with this:
1) Just don't pay; it's tempting, but it could cost you. Instead, prepay and be patient.
2) Pay and use a Double attack; you have a 50% chance of taking back the point you granted your opponent. You essentially dealt an extra point of stamina damage for free.
3) Pay and retreat; your opponent will have to waste an action point closing the gap. This usually leaves them with too few points for a Triple attack.
4) Pay and Triple attack; if your opponent paid and gave you a free action point, make him/her regret it! This is risky, as failing to knock them out means they can try the same.
Prepaying is a great way to deter an enemy. If you already have two points for your upcoming turn, they will hesitate to pay, as this will allow you to start your upcoming turn with a whopping three action points. This is enough to attempt a Triple attack without paying, or any other potent three-point combination. If you're feeling particularly bold or aggressive, you can choose to pay, granting you the fearsome four-point turn.
The Quadruple attack is a force to be reckoned with, as are the other actions you can take with four action points at your disposal. The only way it happens is if you prepay, your opponent pays, and then you pay (each of these grants one action point, a total of three additional points). A four-point turn for either player usually ends the match, as the options available are simply overpowering. Luckily, it can only happen if the opponent allows it.
So why would the opponent allow it? Usually for one of two reasons:
1) He/she is stupid; remedy this immediately with a righteous four-point beatdown.
2) He/she has good reason to believe that the match can end on the current turn, usually by paying and attempting a Triple attack (or a combo that can knock you out from sheer stamina damage). He/she may also be aiming to deplete your upcoming action points with an attempted Double attack.
Use movement to pressure and frustrate your opponent. Samurai can close the gap to zero steps, making ninja apprehensive, whereas ninja can open the gap far wider than two steps, causing impatient samurai to charge in while the ninja prepays and waits. It is always easier to avoid attacks than to land them; avoiding attacks is a simple matter of retreating, whereas landing them involves advancing and attacking. For the same number of points, a character can avoid attacks indefinitely.