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If the holder has 25% HP or less, then it will consume the Lansat Berry, and the effect of Focus Energy takes place (critical hit ratio raised by two stages). This does not occur if Focus Energy is already in effect for the user. Natural Gift becomes an 80 Base Power Flying-type attack when the user holds this Berry, but the Berry is consumed after use.
Playing for critical hits is generally a fool's game, as they will never happen often enough to make them worthwhile over the power you lost by using up your item slot, or using high critical hit-ratio moves, which are usually weaker or less accurate than other moves. Even though Lansat Berry gives the effect of Focus Energy - which on its own increases the chance of a critical hit from stage 1 to stage 3 (or in layman's terms, once in every sixteen hits to once in every four hits), which is a huge boost - it is still not the most useful effect you could have when at 25% of your health, as it will only save your bacon once in every four battles on average - and that's if your opponent does not carry a priority attack to finish you off with.
Theoretically, Pokemon who might find Lansat Berry useful are those with the Super Luck ability, as it gives all of their moves a critical hit stage of at least 4, meaning they will get a critical hit roughly one in every three attacks. Pokemon with the Sniper ability, such as Kingdra and Octillery, can also find Lansat Berry useful because any critical hits from them will deal three times the normal amount of damage instead of two, although this will only happen once in every four attacks on average. However, all of these Pokemon will certainly find other items more useful.
Battle Subway - Talk to a Clerk on the platform after 105 consecutive victories on any train.