It depends upon your strategy, really; different people have different opinions due to different purposes.
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no
Assuming the defending monster is also in attack position, both monsters will be destroyed.
Forbidden Chalice will negate Marshamallon's 'cannot be destroyed by battle' effect. Because Forbidden Chalice changes the ATK of a monster, it can be used in the Damage Step, after Marshmallon is flipped by an attack. If you did this, you will not take the 1000 damage from Marshmallon either - his damage dealing effect activates after Damage Calculation, but will be negated.
Best place to watch any episodes of Yugioh, Yugioh GX, Yugioh 5D's, and Yugioh Zexal is at the website 4kidstv.com
Series after: yugioh 5'ds is yugioh Zexal.