Experience Boost (EXPチャンス, EXP Chansu?), also translated as EXP Chance and EXP Boost, is an ability introduced in Kingdom Hearts II. It doubles the experience the user earns for defeating enemies when HP is below 50% in Kingdom Hearts II, or 25% in Birth by Sleep and Dream Drop Distance.
Experience Boost can be difficult to keep active while farming EXP, as it requires Sora's health to be at risk, but it can also be inadvertently deactivated if Sora picks up HP Prizes from defeated enemies. In order to maintain Sora's HP in the critical range without dying, the player should equip Second Chance, Once More, Damage Control, Defender, apply any available Defense Boosts, and unequip Jackpot, Draw, and Lucky Lucky.
Experience Boost is especially hard to utilize at the beginning and at higher difficulties. A way to utilize it is to beat enemies one by one and then run to a different checkpoint, then back. This way if Sora is killed, the player can retain all the experience gained until that point. This is especially good for critical mode as when Sora has less than half of his total HP at the beginning of the game, most enemies can kill him with one hit. The best way to grind to level 99 is to kill the Nobodies from the first save point in The World That Never Was all the way to The Door to Darkness as the player will level to 99 very quickly. This may be tricky at first, but it gets easier as Sora levels.
Learning EXP Chance
Kingdom Hearts II
Kingdom Hearts Birth by Sleep
Kingdom Hearts 3D: Dream Drop Distance