I have not found anyway to get passed the invulnerability of Propagator in the treasure chest in the developer's house in Narshe.
Anyone has any idea how to defeat this super boss ?

Are you talking about omega mk V? or are you using a alternate means of playing the hacks more recent update. Well, if your trying to beat the superboss in the chest and its named "Omega Mark V" here is my method.

I have a party composed of the most useful characters in the hack, Aurora, Cloud, Arc, Eiko. Each of them plays their part and all have level 99.

Omega mk V uses mighty guard to become invincible, but if you were to check its weakness via scan, you will find out its weak to earth. You have to use quake to slap off its invincibility. But the battle can be a crazy one.

Omega mk V when it loses invincibility will start spamming laser barrage which deals damage based on max hp though I cannot remember how much, it also will use last word which acts like a multitarget dispel. It also loves to use heartless angel, which sometimes actually is a single target use. Also before it reuses mighty guard, it will use hyperdrive, which is instant death if it hits, so here is what you need to do.

Cloud: He is the main attacker, when invincibility is off, have him use omnislash v5, if his speed stat is high enough, he can repeatedly reuse for helpful damage dealing. He should also be maxed out on magic in his magic list so he can use healing and reraise when needed.

Aurora: Should be equipped with zophar shard to use dualcast, which can be used to use reraise on two party members, or quake followed by curaga or lower if your magic power is powerful enough.

Eiko: Same job as aurora but should be equipped with golem esper in case omega mk V decides to get physical.

arc : Will likely have to be the main defense. Have arc equipped with argist, the knife with 30% evade, as well as star shield, and misty coat. for relics, wear the soul booster and aeon crystal, when in combat, have arc use runic, which if timed right can absorb hyperdrive, though no hp is absorbed from hyperdrive.

If I can, I will try to post a video of me defeating omega mark V.

Trolverine : I had the exact same problem. And since Omega Mark 5 is now Propagator, strategy must change. I actually asked Gi Nattak, the mod owner, and Lord Envoy, the script writer for the bosses. This is what Lord Envoy gave me: Time attacks(like Slow or Haste) breaks the Propagator's invulnerability, but when I tried it, Slow and Stop won't work, but Haste works(but this still doesn't render Propagator vulnerable). If you last more longer, Propagator gets berserk and spams Tentacle. Much longer, he shines yellow and uses Silence and Hyperdrive. I gave up on Lord Envoy's response, and I found this: Unfocused attacks are the Propagator's hit-and-counter spots. That means Quake and Meltdown can hit him with hit points, but here is the problem, Meltdown heals it. But, if you use Quake, there is an AWFULLY slim chance it will damage Propagator, and it will always counter with Hyperdrive. Just remember, when Propagator uses Barrier Change, make sure it is weak against earth power, Serin's Psionic Wave Aether also works to add in some weaknesses. Also, do have inherent Float and element-negating equipment. (This is an edited post, IGNORE the method I showed you)The method I gave is impossible to use, so, take a party of 4. The essential member is a Level 70 Cloud. Use Cloud's Limit Move No.7(Finish Touch) against the Propagator twice. Watch the magic. (Note that it must hit, no miss, and it should hit TWICE!)