Activate only when a face-up "Neos" Fusion Monster you control is destroyed. Special Summon 1 "Elemental Hero Neos" from your Deck in Attack Position. It gains 1000 ATK and is destroyed during the End Phase of this turn.
There's no problem with the text.
When the monster is destroyed by battle, it's still in the damage step, and apparently this card can not be activated during the damage step.
After that it can not be activated too, because it already missed timing due to the word "when..."
The only time you can use it is if your "Neos" Fusion was destroyed by card effect outside the damage step, and that effect was the last thing to resolve. (Being an optional "when" effect, it won't work if destroyed in the middle of a chain.)
So basically, it'd have to be a card like Special Hurricane or Widespread Ruin.