You don't seem to understand the importance of substats. In late game, critical hits account for a lot of your damage. The critical stat increases the chance of getting a crit on your enemy and decreases the chance of your enemy landing a crit on you. Injury is also important, as it increases the damage of your crits and decreases the damage you receive from your opponent's crits. Critical and injury are just as if not more important than base stats like resistance and life, so it is worth it to change your assist to Tenten. If you have high enough critical, you can crit on your opponent 80% of the time, which means you deal much more damage. You probably have lower crit and injury than the other guy, so if you increase them, you will also take less damage

Which event is she free in? I don't remember seeing her for free.

Too bad for me XD My server is almost dead and all the active players are f2p or only buy jonin medals