Almost 2K ironite spent in grinding LOL XV, got over 3000 siren frags, and only around 650 Lilith frags... that's enough for a whooping 25 cosmic lilith frags... at this pace, will take more than a year to collect the required 500 frags.
Cosmic lilith frags should be more accessible, perhaps some dropping in LOL XI-XL, or bought with the useless redemption/absolution frags.