Nobody said it will be a double damage.
However her 50% damage increase for your entire dark team + damage cut, that's already good enough to bench her in a burst/damage build rather than Hades.
And I know that isn't a double damage but 20-25% are also good and I don't know if I will ever get a P2W Eidolong, but who cares?
P2W Eidolon will make your life easier but you can also do without them. Don't know if this also applies for tower.
Wait. Shingen offers +200% burst damage and Pluto offers for her self for each lego stack +100% burst damage. Therefore she would just need 3 lego stack to reach 500%, or not?