The key is simpler than you think: Sweepers. You absolutely don't need Summer Moko; in fact, you don't have to bring any Psy units. All you need is AOE attacks via Sweepers, and a reliable Healer.

My team is rather simple: MC, Marian (with signature weapon), S.Motoha (no signature, can be replaced with any Sweeper tbh), Queen (with signature weapon, can also be replaced with any Sweeper), and Riko (with event weapon). For my Personas, no kidding, Leanan Sidhe works perfectly fine for me. Only Queen is well-built stat-wise, but even semi-built characters are fine, as is the case with my Marian and S.Motoha.

One thing to note is that all the enemies will spam Mazionga/Zionga, so do not bring anyone/any Persona that weak to Electricity. This is why Leanan Sidhe is so breezy to use even though she's only Lv 43, because she's not weak to Elec and her main skill, Psychokinetic Disruption, has Def Down for S.Motoha to take advantage of.

The fight's gimmick is simple: You need to kill Raja Naga before being able to kill the other Nagas. The enemy will do this in order

* Raja Naga casts 3 stacks of damage reduction buff to all Nagas, plus one for himself at the start of battle and every turn afterwards.
* Raja Naga targets one member to deliver severe damage at the next turn, unless you hit any Naga 20 times in total.
* If any Naga other than Raja Naga has 25% or more HP by this point, they increase their damage, then heal 50% of their HP. Raja Naga will not heal at all.

So really, you just spam AOE attacks. All the Nagas will have 0 damage reduction in no time, by which point, when Raja Naga targets one of your members, you need to make sure to hit the enemy 20 times. Do note that AOE attacks with multihit effects don't count, so Leanan Sidhe's Psychokinetic Disruption still counts as 5 hits, not 10. Save your Highlight for your Sweeper, not your Healer, because if you don't, you're likely not to be able to reach the required hits to dispel the targeting.

All in all, honestly, a much breezier fight than last update's Genbu, where you kinda have to get S.Motoha. Just get your characters decently built, and you shouldn't face that much trouble. Good luck, y'all!

Raja Naga 4 is pretty easy - A Guide