The Elder Scrolls V: Skyrim

Whoa that's a tricky one.

What I do is sneak attack with daggers to get the neck slit animation (you turn visible exactly when you slit his throat and looks awesome)
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Relax, it's just a game.

Aura whisper or detect spells to know where your enemies are will allow you to time your invisibility casts. Quiet casting should be a forgone conclusion.

You can stay muffled longer than invisible and cheaper magicka wise, so stay muffled mostly. Study the area and situation, and plan your attacks accordingly.

To become invisible as you enter a new cell, charge the spell, open the door and cast the spell during the loading screen. You will become invisible almost immediately upon entering.
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Casually Hardcore

Don't forget bound dagger for that "cool" factor.
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Fascination ends