If you were a warrior who could lag him reliably, then every time he tried casting sleep on you he'd be taking his life into his hands. Only you're a ranger, so he can do it rather safely.
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<br>And getting slept isn't the end of the world, as long as you don't get slept in an area where he can summon you somewhere nasty, and as long as you're prepared to deal with curse/blind/etc.
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<br>Let's say you get the jump on him w/ ambush. At your levels, he's probably at or below half health. Now, if he comes back and tries the sleep, and misses, there's a very real chance he dies. What more can you ask for?
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<br>My advice to you: roll an anti-paladin w/ identical race/cabal to this guy who killed you. See how powerful sleep is from the sleep-er's point of view.