The effect of “Elemental Hero Thunder Giant” targets a monster. Discarding a card for this effect is a cost. This effect can be chained to.
If a monster is equipped with “Megamorph” and its ATK is lower than that of “Elemental Hero Thunder Giant”, and the effect is activated to destroy the equipped monster, but “Mystical Space Typhoon” is chained to destroy “Megamorph”, so that the monster’s original ATK is now higher than the ATK of “Elemental Hero Thunder Giant”, the effect of “Elemental Hero Thunder Giant” disappears and the monster is not destroyed.
Likewise, if the effect of “Elemental Hero Thunder Giant” is activated, targeting “King of the Skull Servants” with 2000 original ATK, and the opponent chains “Curse of Aging” and discards “Skull Servant” to activate it, then “King of the Skull Servants” has 3000 original ATK and is not destroyed.