Fighting Reiko the second time may be tough because of her pesky clones that keeps on spawning. However, you can simply win this battle, as long as you have at least one Cure Gem and a Power Gem.

First, target the real Reiko (the one using a shotgun) then get close to her. Immediately use a Power Gem to increase your attack then use a Counter Stance (Press Y and B at the same time) to deal heavy damage to her. Since Counter Stance only consumes one HP block, you can do this as long as you want. The longer your HP, the better. Just keep activating it as quickly as you can. Aside from damaging her, this attack will also kill nearby clones.

Once she gets low on HP, she'll go on Rampage mode so make sure you heal yourself by then. Wait until she starts attacking before you attack her since she is invulnerable while "changing form"

Using the technique described above, the battle should last for a few seconds only.