Before 4.0, it's a 30% flat damage boost, and not stat boost. If you are using other buffs such as barrier and elemental enhancement, wb asuma boost end up being being a much bigger boost than another +30% like iruka.

Not sure if 4.0 changed this; some buffs got changed in 4.0.

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As long as a ninja is at full hp, wb asuma buff is applied. If you lose hp, you lose the buff. If you leech/heal back to full hp, you regain the buff. The buff go away regardless of health if wb asuma is not alive on the field.