Theists roll their eyes at that, because nobody really thinks their god is omniscient or omnipotent. They may say so, either to deceive the nonbelievers, or out of ignorance of what omnimax really means, but every religion I can think of has had a fallible god, sometimes very fallible. There are the notoriously arrogant Greek gods, the stupidly belligerant Norse gods, the Jewish/Christian god foiled by iron chariots, and deceptive serpents, even Buddhists with their infallible smug asshole of a god have as a saying "If you meet the Buddha on the road, kill him."

Fact of the matter is no god is omni-anything, since that would prove they don't exist, and cannot be believed. Gods don't have to be omniscient. They only have to be way more knowledgable and aware than anyone else. They don't have to be all powerful, only way more powerful than anything mere mortals could muster. So saying "Aha! But your god can't possibly be all powerful, because then he could make a stone that he couldn't lift! Checkmate, theists!" falls flat, in the face of (outside of boasting) doctrine basically saying that their god makes mistakes and can't do everything.

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If we assume that god, by definition, must be omniscient

Why must that be true by definition? Many of the Greek gods were clearly not omniscient, because the stories about them all involve intrigues and hiding things from each-other.

Also, you can't disprove a god's existence by making a logic puzzle that's hard for you to puzzle out. Just because it's a toughie for you doesn't mean that it disproves the existence of gods.

That isn't even a particularly difficult logic puzzle.