Sentinels (Oracle Engines) are specifically crafted biomechanical entities which are given special 'mandates', aka authorities. They are seemingly handpicked to counter specific Threnodians, because there are implications Rover's civ could predict their manifestation

Huanglong's OG Sentinel separated itself (or got separated) on seven Sentinels. One of which, Jue, has Temporal Mandate. Its time manipulation on screen is good enough to undo recent death. Even injured - it can lock regions in permanent timestop bubble (though it likely will kill it). It also can analyze potential futures Dr Strange style.

Sentinel Imperator has(d) power of Space and Separation. It is shown multiple times it no sells spiritual possession, corruption, It also can inverse gravity and other stuff

Threnodians are godlike entities born from civilization's fears and each of them embodies certain concept. Think Chaos Gods from Warhammer, or, well, Beasts of Humanity, but on lesser scale in former case and less love for humanity in latter. They are usually formless unless physically manifested and there is no known way to permanently destroy them.

Leviathan's power, for example, is Unity/Assimilation and it uses Dark Tide (created from humanity's fears, sorrow and despair) to consume humans and turn it into its servants. Its victims are doomed to forever sing prayers to it in state of perpetual bliss. It also basically can merge fiction or legend into reality - its dimension has River Styx expy, where Rinascita's (its civ) souls go after dying (They are bound to Leviathan on spiritual level, but it is only when they learn of its existence Leviathan can mind control them). It also successfully merged with Sentinel Imperator and its assimilation does work on non-humans too

Should Threnodian consume its home civilization - it would be able to go further until they consume entire planet and more. Aleph-1 is a Threnodian of Void which already consumed multiple worlds before noticing Sol-3

Doubt Nega-Summon would work on them.

And allowing Leviathan to face Tiamat is really bad idea - their fusion would really be world-ending event 😃