Synonyms for Bulletproof (same or very similar meaning)
WordNet sense 1 (without flaws or loopholes):
unassailable, unshakable, watertightWordNet sense 2 (incapable of being contested or disputed):
incontestable, incontestibleWordNet sense 3 (protected by armor (used of persons or things military)):
armored, armouredFrom the ODE community, based on WordNetadd/edit