Yes, truly you’ll be able to’t reply to attackers being declared, since declaring attackers doesn’t use the stack (CR508.1). You can reply to the set off from Greatsword being placed on the stack after attackers are declared, however eradicating the supply of the flexibility as soon as it has been triggered/activated doesn’t stop that from resolving (CR113.71). Emphasis within the guidelines quotes mine.
508.1 First, the energetic participant declares attackers. This turn-based motion doesn’t use the stack. To declare attackers, the energetic participant follows the steps under, so as. If at any level throughout the declaration of attackers, the energetic participant is unable to adjust to any of the steps listed under, the declaration is prohibited; the sport returns to the second earlier than the declaration (see rule 727, “Handling Illegal Actions”).
113.7a Once activated or triggered, a capability exists on the stack independently of its supply. Destruction or elimination of the supply after that point received’t have an effect on the flexibility. Note that some talents trigger a supply to do one thing (for instance, “Prodigal Pyromancer deals 1 damage to any target”) moderately than the flexibility doing something instantly. In these circumstances, any activated or triggered capability that references details about the supply to be used whereas asserting an activated capability or placing a triggered capability on the stack checks that data when the flexibility is put onto the stack. Otherwise, it can test that data when it resolves. In each cases, if the supply is not within the zone it’s anticipated to be in at the moment, its final recognized data is used. The supply can nonetheless carry out the motion though it not exists.