You can argue that Helena was defeatable in previous editions of CBN , you just had to make her waste their resources against Menele becuase last time that happened both ended at the verge of death (In fact almost every Character in Chicago by Night 1st-2nd had an enemy or weakness you could exploit to defeat them even if they where more powerful than you).

Now this is just my two cents but Helena is a 3000+ Years Old Monster so it makes sense for the daughter of zeus to be more powerful than the PCs who should use cunning to defeat her or unveil her manipulation during the climax...