In discussing theology questions with other theists, I almost always find that they undervalue omniscience. Omniscience is AMAZINGLY powerful. A few examples of what omniscience implies:
a) One can persuade anyone of anything. IF one knows to the Nth degree the psychology of another individual, and has total mastery of communication skills, then one will be able to find psychological leverage points, and means to use them to convince -- IE that individual WILL be persuadable by an omniscient entity.
b) Likewise, leverage of the physical plane can be optimized. TINY inputs can be used to create macro-scale outcomes. A few specks of dust could be redirected, and be used to nudge a comet ever so slightly near its apex. 100,000 years later, it can intersect a planetary orbit as the planet passes. A bit of water could be slightly concentrated on a slope, and suddenly the drop in static friction leas half a mountainside to let go.
c) Even more drastically, theoretical physicists have discovered that one can do ANYTHING by adding or subtracting terms from quantum equations! As with the minor tweaks above, these can be very low or eve NO energy inputs! All one needs is the power to "put fire in the equations". Add a term locally, in just the right spot, for a nanosecond, and any threat could decorrelate, or collapse into a black hole, or find itself at the other end of a wormhole, or one of innumerable other "tricks" that I have not heard of, or modern physicists have not yet discovered.
So, an omniscient entity, with even limited power, so long as its power allows it to manipulate physics locally on demand, should be able to maintain itself forever against any threat.