The activated ability overwrites all previous effects that set Riptide Mangler’s base power to specific a value (such as previous activations of the ability). Other effects that set its base power that start to apply after Riptide Mangler’s ability resolves will overwrite this effect.

Effects that modify Riptide Mangler’s power, such as the effect of Giant Growth or Glorious Anthem, will apply to Riptide Mangler no matter when they started applying. The same is true for counters that affect Riptide Mangler’s power and effects that switch its power and toughness.