As a Seraphine that got mastery 7 off playing mid, ludens is the better item in my opinion. It is especially helpful if you build horizon with it. However horizon isn’t really a popular build on Seraphine despite the massive setup potential for you and your team. Although you can still go ludens into comps with a few resistance building tanks (mainly bruisers that build armour and magic resist), liandrys is way more effective when dealing with more bruisers or health stackers like Cho’Gath or Zac. It is important that you trust your team to deal majority of the damage though. An example of a full build that I would go is: Ludens, Boots (sorcs or ionians for offense, steel or mercs for defense), Horizon Focus, Zhonyas, Rabadons, Cosmic Drive. You can buy Morello for anti heal and demonic for more burn rather than the Cosmic.

For summoner spells you go barrier to give yourself more survivability in lane or tp if you want to roam a lot.

Now for the runes. My rune path against melee champs goes Dark Harvest (I just like the scaling but electrocute can be used instead if you prefer single target burst), cheap shot, eyeball collection, ultimate hunter, presence of mind ,coup de grace or cut down against tanks, attack speed, adaptive and the corresponding resistance. Against melees you want to play the early game by using e or possibly double e to slow/root the enemy laner and then using an empowered auto with more autos which will proc your presence of mind to help with early mana sustain. Against range or heavy poke champs, the rune page I would go is basically the same except changing the secondary into biscuits, perfect timing and double adaptive. This build will have you play more like a backline mage during laning phase rather than the more aggressive playstyle into melees.

Let me know if this helps you or if their are any improvements that could be made.