I'm a terrible League player and I only play with friends for fun, but I really enjoy the setup I have for Lillia so I thought I'd share it. With the new jungle and mythic items Lillia can become extremely potent with only a few items. My favourite new item combo is Rylai's and Imperial Mandate, as the ability slow added by Rylai's triggers the Mandate and can set you and your teammates up for some big burst damage. This pairing is fairly cheap in the context of Lillia's vaible item pool and only need each other to be effective, it's the quickest and cheapest mythic combo for my Lillia setup. For harder games, start building Demonic Embrace and Liandry's to make Lillia quite swoll and give her a ton of burn damage to whittle away the health of the enemy team.

I'm sure there are more optimal runes for Lillia, but I like being rewarded for going fast and I enjoy the extra mana regen and lifesteal. I use cheap shot mainly for the Rylai's and Imperial Mandate combo, but it also synergises well with some of Lillia's other abilities.

I'm going to explore some other speedy runes with the new items, something like Phase Rush or Predator, so I might add more Rune setups if I find them fun.